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: add squash subcommand to fold a range sequencer: share the squash message marker helpers and flags history: give commit_tree_ext a message template history: extract helper for a commit's parent tree