Merge branch 'en/removing-untracked-fixes'

Various fixes in code paths that move untracked files away to make room.

* en/removing-untracked-fixes:
  Documentation: call out commands that nuke untracked files/directories
  Comment important codepaths regarding nuking untracked files/dirs
  unpack-trees: avoid nuking untracked dir in way of locally deleted file
  unpack-trees: avoid nuking untracked dir in way of unmerged file
  Change unpack_trees' 'reset' flag into an enum
  Remove ignored files by default when they are in the way
  unpack-trees: make dir an internal-only struct
  unpack-trees: introduce preserve_ignored to unpack_trees_options
  read-tree, merge-recursive: overwrite ignored files by default
  checkout, read-tree: fix leak of unpack_trees_options.dir
  t2500: add various tests for nuking untracked files

diff --cc builtin/am.c
Simple merge

diff --cc builtin/clone.c
Simple merge

diff --cc builtin/merge.c
Simple merge

diff --cc builtin/stash.c
Simple merge

diff --cc builtin/submodule--helper.c
Simple merge

diff --cc merge-ort.c
Simple merge

diff --cc merge-recursive.c
Simple merge

diff --cc sequencer.c
Simple merge

diff --cc submodule.c
Simple merge

diff --cc unpack-trees.c
Simple merge