fetch: don't ask for remote HEAD if followRemoteHEAD is "never"

fetch: don't ask for remote HEAD if followRemoteHEAD is "never"

When we are going to consider updating the refs/remotes/*/HEAD symref,
we have to ask the remote side where its HEAD points. But if we know
that the feature is disabled by config, we don't need to bother!

This saves a little bit of work and network communication for the
server. And even a little bit of effort on the client, as our local
set_head() function did a bit of work matching the remote HEAD before
realizing that we're not going to do anything with it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>