for-each-ref: Field with abbreviated objectname

Introduce a :short modifier to objectname which outputs the abbreviated
object name.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/for-each-ref.c

@@ -227,6 +227,9 @@ static void grab_common_values(struct atom_value *val, int deref, struct object
 			strcpy(s, sha1_to_hex(obj->sha1));
 			v->s = s;
 		}
+		else if (!strcmp(name, "objectname:short")) {
+			v->s = find_unique_abbrev(obj->sha1, DEFAULT_ABBREV);
+		}
 	}
 }