Merge branch 'jc/fix-pull-ff-only-when-already-up-to-date'

"git pull --ff-only" and "git pull --rebase --ff-only" should make
it a no-op to attempt pulling from a remote that is behind us, but
instead the command errored out by saying it was impossible to
fast-forward, which may technically be true, but not a useful thing
to diagnose as an error.  This has been corrected.

* jc/fix-pull-ff-only-when-already-up-to-date:
  pull: --ff-only should make it a noop when already-up-to-date

diff --cc builtin/pull.c
Simple merge