apply: set file mode when --reverse creates a deleted file

Commit 01aff0a (apply: correctly reverse patch's pre- and post-image
mode bits, 2023-12-26) revised reverse_patches() to maintain the desired
property that when only one of patch::old_mode and patch::new_mode is
set, the mode will be carried in old_mode. That property is generally
correct, with one notable exception: when creating a file, only new_mode
will be set. Since reversing a deletion results in a creation, new_mode
must be set in that case.

Omitting handling for this case means that reversing a patch that
removes an executable file will not result in the executable permission
being set on the re-created file. Existing test coverage for file modes
focuses only on mode changes of existing files.

Swap old_mode and new_mode in reverse_patches() for what's represented
in the patch as a file deletion, as it is transformed into a file
creation under reversal. This causes git apply --reverse to set the
executable permission properly when re-creating a deleted executable
file.

Add tests ensuring that git apply sets file modes correctly on file
creation, both in the forward and reverse directions.

Signed-off-by: Mark Mentovai <mark@chromium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

apply.c

@@ -2219,7 +2219,7 @@ static void reverse_patches(struct patch *p)
 		struct fragment *frag = p->fragments;
 
 		SWAP(p->new_name, p->old_name);
-		if (p->new_mode)
+		if (p->new_mode || p->is_delete)
 			SWAP(p->new_mode, p->old_mode);
 		SWAP(p->is_new, p->is_delete);
 		SWAP(p->lines_added, p->lines_deleted);