"git stash" rewritten in C.

* ps/stash-in-c: (28 commits)
  tests: add a special setup where stash.useBuiltin is off
  stash: optionally use the scripted version again
  stash: add back the original, scripted `git stash`
  stash: convert `stash--helper.c` into `stash.c`
  stash: replace all `write-tree` child processes with API calls
  stash: optimize `get_untracked_files()` and `check_changes()`
  stash: convert save to builtin
  stash: make push -q quiet
  stash: convert push to builtin
  stash: convert create to builtin
  stash: convert store to builtin
  stash: convert show to builtin
  stash: convert list to builtin
  stash: convert pop to builtin
  stash: convert branch to builtin
  stash: convert drop and clear to builtin
  stash: convert apply to builtin
  stash: mention options in `show` synopsis
  stash: add tests for `git stash show` config
  stash: rename test cases to be more descriptive
  ...