unpack-trees: heed requests to overwrite ignored files

unpack-trees: heed requests to overwrite ignored files

When a directory exists but has only ignored files within it and we are
trying to switch to a branch that has a file where that directory is,
the behavior depends upon --[no]-overwrite-ignore.  If the user wants to
--overwrite-ignore (the default), then we should delete the ignored file
and directory and switch to the new branch.

The code to handle this in verify_clean_subdirectory() in unpack-trees
tried to handle this via paying attention to the exclude_per_dir setting
of the internal dir field.  This came from commit c81935348b ("Fix
switching to a branch with D/F when current branch has file D.",
2007-03-15), which pre-dated 039bc64e88 ("core.excludesfile clean-up",
2007-11-14), and thus did not pay attention to ignore patterns from
other relevant files.  Change it to use setup_standard_excludes() so
that it is also aware of excludes specified in other locations.

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