Merge branch 'hn/branch-prune-merged' into seen

Merge branch 'hn/branch-prune-merged' into seen

"git branch" command learned "--prune-merged" option to remove
local branches that have already been merged to the remote-tracking
branches they track.

* hn/branch-prune-merged:
  branch: add --dry-run for --prune-merged
  branch: add branch.<name>.pruneMerged opt-out
  branch: add --prune-merged <branch>
  branch: prepare delete_branches for a bulk caller
  branch: let delete_branches warn instead of error on bulk refusal
  branch: add --forked <branch>