Merge branch 'jk/ref-cache-non-repository-optim'

The underlying machinery used by "ls-files -o" and other commands
have been taught not to create empty submodule ref cache for a
directory that is not a submodule.  This removes a ton of wasted
CPU cycles.

* jk/ref-cache-non-repository-optim:
resolve_gitlink_ref: ignore non-repository paths
clean: make is_git_repository a public function

diff --cc builtin/clean.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc refs/files-backend.c
Simple merge