Merge branch 'jc/auto-gc-quiet'

Merge branch 'jc/auto-gc-quiet'

Teach "am", "commit", "merge" and "rebase", when they are run with
the "--quiet" option, to pass "--quiet" down to "gc --auto".

* jc/auto-gc-quiet:
  auto-gc: pass --quiet down from am, commit, merge and rebase
  auto-gc: extract a reusable helper from "git fetch"