R1:374f8b10b3cf

Add a "--dry-run" flag to "bin/repository rebuild-identities"

Summary: Ref T13444. Allow the effects of performing an identity rebuild to be previewed without committing to any changes.

Test Plan: Ran "bin/repository rebuild-identities --all-identities" with and without "--dry-run".

Maniphest Tasks: T13444

Differential Revision: https://secure.phabricator.com/D20922