receive-pack: send auto-gc output over sideband 2

Redirect auto-gc output to the sideband such that it is visible to all
clients. As a side effect, all auto-gc error messages are now prefixed
with "remote: " before being printed to stderr on the client-side which
makes it easier to understand that those error messages originate from
the server.

Signed-off-by: Lukas Fleischer <lfleischer@lfos.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index c8e32b2..4abfa38 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -1788,9 +1788,20 @@
 			const char *argv_gc_auto[] = {
 				"gc", "--auto", "--quiet", NULL,
 			};
-			int opt = RUN_GIT_CMD | RUN_COMMAND_STDOUT_TO_STDERR;
+			struct child_process proc = CHILD_PROCESS_INIT;
+
+			proc.no_stdin = 1;
+			proc.stdout_to_stderr = 1;
+			proc.err = use_sideband ? -1 : 0;
+			proc.git_cmd = 1;
+			proc.argv = argv_gc_auto;
+
 			close_all_packs();
-			run_command_v_opt(argv_gc_auto, opt);
+			if (!start_command(&proc)) {
+				if (use_sideband)
+					copy_to_sideband(proc.err, -1, NULL);
+				finish_command(&proc);
+			}
 		}
 		if (auto_update_server_info)
 			update_server_info(0);