apply: correctly reverse patch's pre- and post-image mode bits

When parsing the patch header, unless it is a patch that changes
file modes, we only read the mode bits into the .old_mode member of
the patch structure and leave .new_mode member as initialized, i.e.,
to 0.  Later when we need the original mode bits, we consult .old_mode.

However, reverse_patches() that is used to swap the names and modes
of the preimage and postimage files is not aware of this convention,
leading the .old_mode to be 0 while the mode we read from the patch
is left in .new_mode.

Only swap .old_mode and .new_mode when .new_mode is not 0 (i.e. we
saw a patch that modifies the filemode and know what the new mode
is).  When .new_mode is set to 0, it means the preimage and the
postimage files have the same mode (which is in the .old_mode member)
and when applying such a patch in reverse, the value in .old_mode is
what we expect the (reverse-) preimage file to have.

Reported-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

apply.c

@@ -2220,7 +2220,8 @@ static void reverse_patches(struct patch *p)
 		struct fragment *frag = p->fragments;
 
 		SWAP(p->new_name, p->old_name);
-		SWAP(p->new_mode, p->old_mode);
+		if (p->new_mode)
+			SWAP(p->new_mode, p->old_mode);
 		SWAP(p->is_new, p->is_delete);
 		SWAP(p->lines_added, p->lines_deleted);
 		SWAP(p->old_oid_prefix, p->new_oid_prefix);
@@ -3780,9 +3781,8 @@ static int check_preimage(struct apply_state *state,
 
 	if (!state->cached && !previous) {
 		if (!trust_executable_bit)
-			st_mode = (*ce && (*ce)->ce_mode) ? (*ce)->ce_mode :
-				(state->apply_in_reverse
-				 ? patch->new_mode : patch->old_mode);
+			st_mode = (*ce && (*ce)->ce_mode)
+				? (*ce)->ce_mode : patch->old_mode;
 		else
 			st_mode = ce_mode_from_stat(*ce, st->st_mode);
 	}