fetch: skip tasks related to fetching objects

cmd_fetch() does the following with the assumption that objects are
fetched:

* Run gc
* Write commit graphs (if enabled by fetch.writeCommitGraph=true)

However, neither of these tasks makes sense if objects are not fetched
e.g. `git fetch --negotiate-only` never fetches objects.

Speed up cmd_fetch() by bailing out early if we know for certain that
objects will not be fetched. cmd_fetch() can bail out early whenever
objects are not fetched, but for now this only considers
--negotiate-only.

The same optimization does not apply to `git fetch --dry-run` because
that actually fetches objects; the dry run refers to not updating refs.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -2133,6 +2133,17 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 		strvec_clear(&options);
 	}
 
+	/*
+	 * Skip irrelevant tasks because we know objects were not
+	 * fetched.
+	 *
+	 * NEEDSWORK: as a future optimization, we can return early
+	 * whenever objects were not fetched e.g. if we already have all
+	 * of them.
+	 */
+	if (negotiate_only)
+		goto cleanup;
+
 	prepare_repo_settings(the_repository);
 	if (fetch_write_commit_graph > 0 ||
 	    (fetch_write_commit_graph < 0 &&