fetch: avoid ls-refs only to ask for HEAD symref update

When we fetch from a configured remote, we may try to update the local refs/remotes/<origin>/HEAD, and so we ask the server to advertise its HEAD to us. But if we aren't otherwise asking about any refs at all, then we know this HEAD update can never happen! To consider a new value for HEAD, the set_head() function uses guess_remote_head(). And even if it sees an explicit symref value for HEAD, it will only report that as a match if we also saw that remote ref advertised, and it mapped to a local tracking ref via get_fetch_map(). In other words, a fetch like this: git fetch origin $exact_oid:refs/heads/foo can never update HEAD, because we will never have fetched (nor even see the advertisement for) the ref that HEAD points to. Currently the command above will still call ls-refs to ask about the HEAD, even though it is pointless. This patch teaches it to skip the ls-refs call entirely in this case, which avoids a round-trip to the server. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>