Update the untracked cache subsystem and change its primary UI from
"git update-index" to "git config".

* cc/untracked:
  t7063: add tests for core.untrackedCache
  test-dump-untracked-cache: don't modify the untracked cache
  config: add core.untrackedCache
  dir: simplify untracked cache "ident" field
  dir: add remove_untracked_cache()
  dir: add {new,add}_untracked_cache()
  update-index: move 'uc' var declaration
  update-index: add untracked cache notifications
  update-index: add --test-untracked-cache
  update-index: use enum for untracked cache options
  dir: free untracked cache when removing it