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

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

The 'git branch' command has been taught the '--delete-merged' option
to remove local branches that are already merged into their tracked
remote-tracking branches.

* hn/branch-delete-merged:
  branch: add --dry-run for --delete-merged
  branch: add branch.<name>.deleteMerged opt-out
  branch: add --delete-merged <pattern>
  branch: prepare delete_branches for a bulk caller
  branch: let delete_branches skip unmerged branches on bulk refusal
  branch: convert delete_branches() to a flags argument
  branch: add --forked filter for --list mode