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: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

diff --git a/commit.c b/commit.c
index 6393e1b..a51d2cd 100644
--- a/commit.c
+++ b/commit.c
@@ -225,7 +225,7 @@
 	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--;