Add an administrative `bin/repository importing` command to list importing commits

Summary: Ref T4068. Adds a command to list all commits in an "importing" status. This will allow users to use `reparse.php` to diagnose and repair issues.

Test Plan:
  - Ran `bin/repository importing P`, etc.
  - Used `reparse.php` to reparse some commit stages and saw status update correctly.
  - Ran on a repo with no importing commits.
  - Ran with `... --simple | xargs`, which saves us having to put an `awk` or something in there for users.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4068

Differential Revision: https://secure.phabricator.com/D7515