unpack-trees: don't perform any index operation if we're not merging

src[0] points to the index entry in the merge case and to the first
tree to unpack in the non-merge case.  We only want to mark the index
entry, so check first if we're merging.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -772,7 +772,7 @@ static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, str
 	if (unpack_nondirectories(n, mask, dirmask, src, names, info) < 0)
 		return -1;
 
-	if (src[0]) {
+	if (o->merge && src[0]) {
 		if (ce_stage(src[0]))
 			mark_ce_used_same_name(src[0], o);
 		else