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):