Simplify parse_pathspec() codepath and stop it from looking at the
default in-core index.

* bw/pathspec-sans-the-index:
  pathspec: convert find_pathspecs_matching_against_index to take an index
  pathspec: remove PATHSPEC_STRIP_SUBMODULE_SLASH_CHEAP
  ls-files: prevent prune_cache from overeagerly pruning submodules
  pathspec: remove PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE flag
  submodule: add die_in_unpopulated_submodule function
  pathspec: provide a more descriptive die message