Merge branch 'fg/remote-prune'

* fg/remote-prune:
  add tests for remote groups
  git remote update: Fallback to remote if group does not exist
  remote: New function remote_is_configured()
  git remote update: Report error for non-existing groups
  git remote update: New option --prune
  builtin-remote.c: Split out prune_remote as a separate function.

diff --cc remote.c
Simple merge