Merge branch 'hn/history-squash' into seen

Merge branch 'hn/history-squash' into seen

The experimental 'git history' command has been taught a new 'squash'
subcommand to fold a range of commits into a single commit, with any
descendants replayed on top.

* hn/history-squash:
  history: re-edit a squash with every message
  sequencer: share the squash message marker helpers and flags
  history: add squash subcommand to fold a range
  history: give commit_tree_ext a message template
  history: extract helper for a commit's parent tree