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>

builtin/rev-list.c

@@ -38,7 +38,7 @@ static const char rev_list_usage[] =
 "    --tags\n"
 "    --remotes\n"
 "    --stdin\n"
-"    --exclude-hidden=[receive|uploadpack]\n"
+"    --exclude-hidden=[fetch|receive|uploadpack]\n"
 "    --quiet\n"
 "  ordering output:\n"
 "    --topo-order\n"