[PATCH v2] Improve performance of git status --ignored

This is the second version of my patches to improve handling of ignored files

I have decided to break the original patch series into two parts:

 1) Perf improvements to handling ignored directories

 2) Expose extra options to control which ignored files are displayed
 by git status.

This patch will address #1, and I will follow up with another patch for #2.

This patch improves the performance of 'git status --ignored' by cutting out unnecessary work when determining if an ignored directory is empty or not. The current logic will recursively enumerate through all contents of an ignored directory to determine whether it is empty or not. The new logic will return after the 1st file is encountered. This can result in a significant speedup in work dirs with a lot of files in ignored directories.

As an example of the performance improvement, here is a representative example of a repository with ~190,000 files in ~400 ignored directories:

| Command                    | Time (s) |
| -------------------------- | -------- |
| git status                 |    1.2   |
| git status --ignored (old) |    3.9   |
| git status --ignored (new) |    1.4   |

Jameson Miller (1):
  Improve performance of git status --ignored

 dir.c | 47 +++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 41 insertions(+), 6 deletions(-)