Clean up the internal of the name-hash mechanism used to work
around case insensitivity on some filesystems to cleanly fix a
long-standing API glitch where the caller of cache_name_exists()
that ask about a directory with a counted string was required to
have '/' at one location past the end of the string.

* es/name-hash-no-trailing-slash-in-dirs:
  dir: revert work-around for retired dangerous behavior
  name-hash: stop storing trailing '/' on paths in index_state.dir_hash
  employ new explicit "exists in index?" API
  name-hash: refactor polymorphic index_name_exists()