fetch: avoid locking issues between fetch.jobs/fetch.writeCommitGraph

When both `fetch.jobs` and `fetch.writeCommitGraph` is set, we currently
try to write the commit graph in each of the concurrent fetch jobs,
which frequently leads to error messages like this one:

fatal: Unable to create '.../.git/objects/info/commit-graphs/commit-graph-chain.lock': File exists.

Let's avoid this by holding off from writing the commit graph until all
fetch jobs are done.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -1602,7 +1602,8 @@ static int fetch_multiple(struct string_list *list, int max_children)
 			return errcode;
 	}
 
-	argv_array_pushl(&argv, "fetch", "--append", "--no-auto-gc", NULL);
+	argv_array_pushl(&argv, "fetch", "--append", "--no-auto-gc",
+			"--no-write-commit-graph", NULL);
 	add_options_to_argv(&argv);
 
 	if (max_children != 1 && list->nr != 1) {