Merge branch 'ps/history-drop' into seen

Merge branch 'ps/history-drop' into seen

The experimental "git history" command has been taught a new "drop"
subcommand to remove a commit and replay its descendants onto its
parent.

* ps/history-drop:
  builtin/history: implement "drop" subcommand
  builtin/history: split handling of ref updates into two phases
  reset: stop assuming that the caller passes in a clean index
  reset: allow the caller to specify the current HEAD object
  reset: introduce ability to skip reference updates
  reset: introduce dry-run mode
  reset: modernize flags passed to `reset_head()`
  reset: drop `USE_THE_REPOSITORY_VARIABLE`
  read-cache: split out function to drop unmerged entries to stage 0