auto-gc: extract a reusable helper from "git fetch"

Back in 1991006c (fetch: convert argv_gc_auto to struct argv_array,
2014-08-16), we taught "git fetch --quiet" to pass the "--quiet"
option down to "gc --auto".  This issue, however, is not limited to
"fetch":

    $ git grep -e 'gc.*--auto' \*.c

finds hits in "am", "commit", "merge", and "rebase" and these
commands do not pass "--quiet" down to "gc --auto" when they
themselves are told to be quiet.

As a preparatory step, let's introduce a helper function
run_auto_gc(), that the caller can pass a boolean "quiet",
and redo the fix to "git fetch" using the helper.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -1864,3 +1864,16 @@ int run_processes_parallel_tr2(int n, get_next_task_fn get_next_task,
 
 	return result;
 }
+
+int run_auto_gc(int quiet)
+{
+	struct argv_array argv_gc_auto = ARGV_ARRAY_INIT;
+	int status;
+
+	argv_array_pushl(&argv_gc_auto, "gc", "--auto", NULL);
+	if (quiet)
+		argv_array_push(&argv_gc_auto, "--quiet");
+	status = run_command_v_opt(argv_gc_auto.argv, RUN_GIT_CMD);
+	argv_array_clear(&argv_gc_auto);
+	return status;
+}