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>

t/t5510-fetch.sh

@@ -1171,6 +1171,15 @@ test_expect_success '--no-show-forced-updates' '
 	)
 '
 
+for section in fetch transfer
+do
+	test_expect_success "$section.hideRefs affects connectivity check" '
+		GIT_TRACE="$PWD"/trace git -c $section.hideRefs=refs -c \
+			$section.hideRefs="!refs/tags/" fetch &&
+		grep "git rev-list .*--exclude-hidden=fetch" trace
+	'
+done
+
 setup_negotiation_tip () {
 	SERVER="$1"
 	URL="$2"