Merge branch 'ab/fetch-negotiate-segv-fix'

Code recently added to support common ancestry negotiation during
"git push" did not sanity check its arguments carefully enough.

* ab/fetch-negotiate-segv-fix:
  fetch: fix segfault in --negotiate-only without --negotiation-tip=*
  fetch: document the --negotiate-only option
  send-pack.c: move "no refs in common" abort earlier

diff --cc builtin/fetch.c
Simple merge