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