builtin/grep: fix leaking object context

Even when `get_oid_with_context()` fails it may have allocated some data
in the object context. But we do not release it in git-grep(1) when the
call fails, leading to a memory leak. Plug it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -1133,6 +1133,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 					 &oid, &oc)) {
 			if (seen_dashdash)
 				die(_("unable to resolve revision: %s"), arg);
+			object_context_release(&oc);
 			break;
 		}