Merge branch 'kb/status-ignored-optim' into next

Merge branch 'kb/status-ignored-optim' into next

"git status --ignored" had many corner case bugs.  Also the command
has been optimized by taking advantage of the fact that paths that
are already known to the index do not have to be checked against
the .gitignore mechanism most of the time.

* kb/status-ignored-optim:
  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 and factor out parts of last_exclude_matching
  dir.c: git-status --ignored: don't list empty directories as ignored
  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