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