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>