unpack-trees: FLEX_ARRAY fix

In unpack-trees.c (line 593), we do

	..
	if (same(old, merge)) {
		*merge = *old;
	} else {
	..

and that "merge" is a cache_entry pointer. If we have a non-zero
FLEX_ARRAY size, it will cause us to copy the first few bytes of the
name too.

That is technically wrong even for FLEX_ARRAY being 1, but you'll never
notice, since the filenames should always be the same with the current
code.  But if we do the same thing for a rename, we'd be screwed.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -590,7 +590,7 @@ static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
 		 * a match.
 		 */
 		if (same(old, merge)) {
-			*merge = *old;
+			memcpy(merge, old, offsetof(struct cache_entry, name));
 		} else {
 			verify_uptodate(old, o);
 			invalidate_ce_path(old);