Merge branch 'jk/fetch-always-update-tracking'

"git fetch origin master" unlike "git fetch origin" or "git fetch"
did not update "refs/remotes/origin/master"; this was an early
design decision to keep the update of remote tracking branches
predictable, but in practice it turns out that people find it more
convenient to opportunisticly update them whenever we have a chance,
and we have been updating them when we run "git push" which already
breaks the original "predictability" anyway.

Now such a fetch does update refs/remotes/origin/master.

* jk/fetch-always-update-tracking:
  fetch: don't try to update unfetched tracking refs
  fetch: opportunistically update tracking refs
  refactor "ref->merge" flag
  fetch/pull doc: untangle meaning of bare <ref>
  t5510: start tracking-ref tests from a known state

diff --cc cache.h
Simple merge