An earlier update made it possible to use an on-stack in-core
lockfile structure (as opposed to having to deliberately leak an
on-heap one).  Many codepaths have been updated to take advantage
of this new facility.

* ma/lockfile-fixes:
  read_cache: roll back lock in `update_index_if_able()`
  read-cache: leave lock in right state in `write_locked_index()`
  read-cache: drop explicit `CLOSE_LOCK`-flag
  cache.h: document `write_locked_index()`
  apply: remove `newfd` from `struct apply_state`
  apply: move lockfile into `apply_state`
  cache-tree: simplify locking logic
  checkout-index: simplify locking logic
  tempfile: fix documentation on `delete_tempfile()`
  lockfile: fix documentation on `close_lock_file_gently()`
  treewide: prefer lockfiles on the stack
  sha1_file: do not leak `lock_file`