rebase: do not remove untracked files on checkout

rebase: do not remove untracked files on checkout

If "git rebase [--apply|--merge] <upstream> <branch>" detects that
<upstream> is an ancestor of <branch> then it will fast-forward and
checkout <branch>. Normally a checkout or picking a commit during a
rebase will refuse to overwrite untracked files, however rebase does
overwrite untracked files when checking out <branch>.

The fix is to only set reset in `unpack_tree_opts` if flags contains
`RESET_HEAD_HARD`. t5403 may seem like an odd home for the new test
but it will be extended in the next commit to check that the
post-checkout hook is not run when the checkout fails.

The test for `!detach_head` dates back to the
original implementation of reset_head() in
ac7f467fef ("builtin/rebase: support running "git rebase <upstream>"",
2018-08-07) and was correct until e65123a71d
("builtin rebase: support `git rebase <upstream> <switch-to>`",
2018-09-04) started using reset_head() to checkout <switch-to> when
fast-forwarding.

Note that 480d3d6bf9 ("Change unpack_trees' 'reset' flag into an
enum", 2021-09-27) also fixes this bug as it changes reset_head() to
never remove untracked files. I think this fix is still worthwhile as
it makes it clear that the same settings are used for detached and
non-detached checkouts.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>