fix memcpy of overlapping area

Caught by valgrind in t5500, but it is pretty obvious from
reading the code that this is shifting elements of an array
to the left, which needs memmove.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

commit.c

@@ -225,7 +225,7 @@ int unregister_shallow(const unsigned char *sha1)
 	if (pos < 0)
 		return -1;
 	if (pos + 1 < commit_graft_nr)
-		memcpy(commit_graft + pos, commit_graft + pos + 1,
+		memmove(commit_graft + pos, commit_graft + pos + 1,
 				sizeof(struct commit_graft *)
 				* (commit_graft_nr - pos - 1));
 	commit_graft_nr--;