Merge branch 'ab/retire-refs-unused-funcs'

Code cleanup.

* ab/retire-refs-unused-funcs:
  refs/ref-cache.[ch]: remove "incomplete" from create_dir_entry()
  refs/ref-cache.c: remove "mkdir" parameter from find_containing_dir()
  refs/ref-cache.[ch]: remove unused add_ref_entry()
  refs/ref-cache.[ch]: remove unused remove_entry_from_dir()
  refs.[ch]: remove unused ref_storage_backend_exists()

diff --cc refs.c
Simple merge

diff --cc refs.h
Simple merge

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