fetch: support hideRefs to speed up connectivity checks

With roughly 800 remotes all fetching into their own
refs/remotes/$REMOTE/* island, the connectivity check[1] gets
expensive for each fetch on systems which lack sufficient RAM to
cache objects.

To do a no-op fetch on one $REMOTE out of hundreds, hideRefs now
allows the no-op fetch to take ~30 seconds instead of ~20 minutes
on a noisy, RAM-constrained machine (localhost, so no network latency):

   git -c fetch.hideRefs=refs \
	-c fetch.hideRefs='!refs/remotes/$REMOTE/' \
	fetch $REMOTE

[1] `git rev-list --objects --stdin --not --all --quiet --alternate-refs'

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/builtin/fetch.c b/builtin/fetch.c
index a09606b472..edbeb19d3b 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1132,6 +1132,7 @@ static int store_updated_refs(const char *raw_url, const char *remote_name,
 	if (!connectivity_checked) {
 		struct check_connected_options opt = CHECK_CONNECTED_INIT;
 
+		opt.exclude_hidden_refs_section = "fetch";
 		rm = ref_map;
 		if (check_connected(iterate_ref_map, &rm, &opt)) {
 			rc = error(_("%s did not send all necessary objects\n"), url);
@@ -1325,6 +1326,7 @@ static int check_exist_and_connected(struct ref *ref_map)
 	}
 
 	opt.quiet = 1;
+	opt.exclude_hidden_refs_section = "fetch";
 	return check_connected(iterate_ref_map, &rm, &opt);
 }