t2500: add various tests for nuking untracked files

Noting that unpack_trees treats reset=1 & update=1 as license to nuke
untracked files, I looked for code paths that use this combination and
tried to generate testcases which demonstrated unintentional loss of
untracked files and directories.  I found several.

I also include testcases for `git reset --{hard,merge,keep}`.  A hard
reset is perhaps the most direct test of unpack_tree's reset=1 behavior,
but we cannot make `git reset --hard` preserve untracked files without
some migration work.

Also, the two commands `checkout --force` (because of the --force) and
`read-tree --reset` (because it's plumbing and we need to keep it
backward compatible) were left out as we expect those to continue
removing untracked files and directories.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/t/t2500-untracked-overwriting.sh b/t/t2500-untracked-overwriting.sh
new file mode 100755 (executable)
index 0000000..2412d12
--- /dev/null
+++ b/t/t2500-untracked-overwriting.sh
@@ -0,0 +1,244 @@
+#!/bin/sh
+
+test_description='Test handling of overwriting untracked files'
+
+. ./test-lib.sh
+
+test_setup_reset () {
+       git init reset_$1 &&
+       (
+               cd reset_$1 &&
+               test_commit init &&
+
+               git branch stable &&
+               git branch work &&
+
+               git checkout work &&
+               test_commit foo &&
+
+               git checkout stable
+       )
+}
+
+test_expect_success 'reset --hard will nuke untracked files/dirs' '
+       test_setup_reset hard &&
+       (
+               cd reset_hard &&
+               git ls-tree -r stable &&
+               git log --all --name-status --oneline &&
+               git ls-tree -r work &&
+
+               mkdir foo.t &&
+               echo precious >foo.t/file &&
+               echo foo >expect &&
+
+               git reset --hard work &&
+
+               # check that untracked directory foo.t/ was nuked
+               test_path_is_file foo.t &&
+               test_cmp expect foo.t
+       )
+'
+
+test_expect_success 'reset --merge will preserve untracked files/dirs' '
+       test_setup_reset merge &&
+       (
+               cd reset_merge &&
+
+               mkdir foo.t &&
+               echo precious >foo.t/file &&
+               cp foo.t/file expect &&
+
+               test_must_fail git reset --merge work 2>error &&
+               test_cmp expect foo.t/file &&
+               grep "Updating .foo.t. would lose untracked files" error
+       )
+'
+
+test_expect_success 'reset --keep will preserve untracked files/dirs' '
+       test_setup_reset keep &&
+       (
+               cd reset_keep &&
+
+               mkdir foo.t &&
+               echo precious >foo.t/file &&
+               cp foo.t/file expect &&
+
+               test_must_fail git reset --merge work 2>error &&
+               test_cmp expect foo.t/file &&
+               grep "Updating.*foo.t.*would lose untracked files" error
+       )
+'
+
+test_setup_checkout_m () {
+       git init checkout &&
+       (
+               cd checkout &&
+               test_commit init &&
+
+               test_write_lines file has some >filler &&
+               git add filler &&
+               git commit -m filler &&
+
+               git branch stable &&
+
+               git switch -c work &&
+               echo stuff >notes.txt &&
+               test_write_lines file has some words >filler &&
+               git add notes.txt filler &&
+               git commit -m filler &&
+
+               git checkout stable
+       )
+}
+
+test_expect_failure 'checkout -m does not nuke untracked file' '
+       test_setup_checkout_m &&
+       (
+               cd checkout &&
+
+               # Tweak filler
+               test_write_lines this file has some >filler &&
+               # Make an untracked file, save its contents in "expect"
+               echo precious >notes.txt &&
+               cp notes.txt expect &&
+
+               test_must_fail git checkout -m work &&
+               test_cmp expect notes.txt
+       )
+'
+
+test_setup_sequencing () {
+       git init sequencing_$1 &&
+       (
+               cd sequencing_$1 &&
+               test_commit init &&
+
+               test_write_lines this file has some words >filler &&
+               git add filler &&
+               git commit -m filler &&
+
+               mkdir -p foo/bar &&
+               test_commit foo/bar/baz &&
+
+               git branch simple &&
+               git branch fooey &&
+
+               git checkout fooey &&
+               git rm foo/bar/baz.t &&
+               echo stuff >>filler &&
+               git add -u &&
+               git commit -m "changes" &&
+
+               git checkout simple &&
+               echo items >>filler &&
+               echo newstuff >>newfile &&
+               git add filler newfile &&
+               git commit -m another
+       )
+}
+
+test_expect_failure 'git rebase --abort and untracked files' '
+       test_setup_sequencing rebase_abort_and_untracked &&
+       (
+               cd sequencing_rebase_abort_and_untracked &&
+               git checkout fooey &&
+               test_must_fail git rebase simple &&
+
+               cat init.t &&
+               git rm init.t &&
+               echo precious >init.t &&
+               cp init.t expect &&
+               git status --porcelain &&
+               test_must_fail git rebase --abort &&
+               test_cmp expect init.t
+       )
+'
+
+test_expect_failure 'git rebase fast forwarding and untracked files' '
+       test_setup_sequencing rebase_fast_forward_and_untracked &&
+       (
+               cd sequencing_rebase_fast_forward_and_untracked &&
+               git checkout init &&
+               echo precious >filler &&
+               cp filler expect &&
+               test_must_fail git rebase init simple &&
+               test_cmp expect filler
+       )
+'
+
+test_expect_failure 'git rebase --autostash and untracked files' '
+       test_setup_sequencing rebase_autostash_and_untracked &&
+       (
+               cd sequencing_rebase_autostash_and_untracked &&
+               git checkout simple &&
+               git rm filler &&
+               mkdir filler &&
+               echo precious >filler/file &&
+               cp filler/file expect &&
+               git rebase --autostash init &&
+               test_path_is_file filler/file
+       )
+'
+
+test_expect_failure 'git stash and untracked files' '
+       test_setup_sequencing stash_and_untracked_files &&
+       (
+               cd sequencing_stash_and_untracked_files &&
+               git checkout simple &&
+               git rm filler &&
+               mkdir filler &&
+               echo precious >filler/file &&
+               cp filler/file expect &&
+               git status --porcelain &&
+               git stash push &&
+               git status --porcelain &&
+               test_path_is_file filler/file
+       )
+'
+
+test_expect_failure 'git am --abort and untracked dir vs. unmerged file' '
+       test_setup_sequencing am_abort_and_untracked &&
+       (
+               cd sequencing_am_abort_and_untracked &&
+               git format-patch -1 --stdout fooey >changes.mbox &&
+               test_must_fail git am --3way changes.mbox &&
+
+               # Delete the conflicted file; we will stage and commit it later
+               rm filler &&
+
+               # Put an unrelated untracked directory there
+               mkdir filler &&
+               echo foo >filler/file1 &&
+               echo bar >filler/file2 &&
+
+               test_must_fail git am --abort 2>errors &&
+               test_path_is_dir filler &&
+               grep "Updating .filler. would lose untracked files in it" errors
+       )
+'
+
+test_expect_failure 'git am --skip and untracked dir vs deleted file' '
+       test_setup_sequencing am_skip_and_untracked &&
+       (
+               cd sequencing_am_skip_and_untracked &&
+               git checkout fooey &&
+               git format-patch -1 --stdout simple >changes.mbox &&
+               test_must_fail git am --3way changes.mbox &&
+
+               # Delete newfile
+               rm newfile &&
+
+               # Put an unrelated untracked directory there
+               mkdir newfile &&
+               echo foo >newfile/file1 &&
+               echo bar >newfile/file2 &&
+
+               # Change our mind about resolutions, just skip this patch
+               test_must_fail git am --skip 2>errors &&
+               test_path_is_dir newfile &&
+               grep "Updating .newfile. would lose untracked files in it" errors
+       )
+'
+
+test_done