for-each-ref: fix objectname:short bug

When objectname:short was introduced, it forgot to copy the result of
find_unique_abbrev. Because the result of find_unique_abbrev is a
pointer to static buffer, this resulted in the same value being
substituted in for each ref.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/for-each-ref.c

diff --git a/builtin/for-each-ref.c b/builtin/for-each-ref.c
index a2b28c6..89e75c6 100644
--- a/builtin/for-each-ref.c
+++ b/builtin/for-each-ref.c
@@ -228,7 +228,8 @@
 			v->s = s;
 		}
 		else if (!strcmp(name, "objectname:short")) {
-			v->s = find_unique_abbrev(obj->sha1, DEFAULT_ABBREV);
+			v->s = xstrdup(find_unique_abbrev(obj->sha1,
+							  DEFAULT_ABBREV));
 		}
 	}
 }