Fixes a handful of issues in the code to traverse working tree to
find untracked and/or ignored files, cleans up and optimizes the
codepath in general.
* kb/status-ignored-optim-2:
dir.c: git-status --ignored: don't scan the work tree twice
dir.c: git-status --ignored: don't scan the work tree three times
dir.c: git-status: avoid is_excluded checks for tracked files
dir.c: replace is_path_excluded with now equivalent is_excluded API
dir.c: unify is_excluded and is_path_excluded APIs
dir.c: move prep_exclude
dir.c: factor out parts of last_exclude_matching for later reuse
dir.c: git-clean -d -X: don't delete tracked directories
dir.c: make 'git-status --ignored' work within leading directories
dir.c: git-status --ignored: don't list empty directories as ignored
dir.c: git-ls-files --directories: don't hide empty directories
dir.c: git-status --ignored: don't list empty ignored directories
dir.c: git-status --ignored: don't list files in ignored directories
dir.c: git-status --ignored: don't drop ignored directories