Merge branch 'ps/history-split'

Merge branch 'ps/history-split'

"git history" learned the "split" subcommand.

* ps/history-split:
  builtin/history: implement "split" subcommand
  builtin/history: split out extended function to create commits
  cache-tree: allow writing in-memory index as tree
  add-patch: allow disabling editing of hunks
  add-patch: add support for in-memory index patching
  add-patch: remove dependency on "add-interactive" subsystem
  add-patch: split out `struct interactive_options`
  add-patch: split out header from "add-interactive.h"