Ignore unreachable commits when testing if a repository has imported

Summary:
Fixes T11309. When checking if a repository was fully imported, we incorrectly allow unreachable, un-imported commits to prevent the repository from moving to "Imported".

This can happen if you delete branches from a repository while it is importing.

Instead, ignore unreachable commits when checking for remaining imports, and when reporting status via bin/repository importing.

Test Plan:

• Stopped daemons.
• Created a new repository and activated it.
• Ran bin/repository update Rxx.
• Deleted a branch in the repository.
• Ran bin/repository update Rxx.
• Ran daemons to flush queue.

Now:

• Ran bin/repository importing. Old behavior: showed unreachable commits as importing. New behavior: does not show unreachable commits.
• Ran bin/repository update. Old behavior: failed to move repository to "imported" status. New behavior: correctly moves repository to "imported" status.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11309

Differential Revision: https://secure.phabricator.com/D16269