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>

t/t2021-checkout-overwrite.sh

@@ -69,4 +69,15 @@ test_expect_success SYMLINKS 'checkout -f must not follow symlinks when removing
 	test_path_is_file untracked/f
 '
 
+test_expect_success 'checkout --overwrite-ignore should succeed if only ignored files in the way' '
+	git checkout -b df_conflict &&
+	test_commit contents some_dir &&
+	git checkout start &&
+	mkdir some_dir &&
+	echo autogenerated information >some_dir/ignore &&
+	echo ignore >.git/info/exclude &&
+	git checkout --overwrite-ignore df_conflict &&
+	! test_path_is_dir some_dir
+'
+
 test_done

unpack-trees.c

@@ -2337,7 +2337,7 @@ static int verify_clean_subdirectory(const struct cache_entry *ce,
 
 	memset(&d, 0, sizeof(d));
 	if (o->dir)
-		d.exclude_per_dir = o->dir->exclude_per_dir;
+		setup_standard_excludes(&d);
 	i = read_directory(&d, o->src_index, pathbuf, namelen+1, NULL);
 	dir_clear(&d);
 	free(pathbuf);