Add an "--importing" flag to bin/repository reparse

Summary: Fixes T6839. Sometimes, worker tasks go astray for whatever reason. This automates the step of bin/repository importing | xargs | mangle mangle | bin/repostiory reparse.

Test Plan: Ran various flavors of the command, got good looking results.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6839

Differential Revision: https://secure.phabricator.com/D13362