Merge branch 'ps/maintenance-missing-tasks'

Merge branch 'ps/maintenance-missing-tasks'

Make repository clean-up tasks "gc" can do available to "git
maintenance" front-end.

* ps/maintenance-missing-tasks:
  builtin/maintenance: introduce "rerere-gc" task
  builtin/gc: move rerere garbage collection into separate function
  builtin/maintenance: introduce "worktree-prune" task
  builtin/gc: move pruning of worktrees into a separate function
  builtin/gc: remove global variables where it is trivial to do
  builtin/gc: fix indentation of `cmd_gc()` parameters