R1:75fa580f3f3f

Add PHIDs to DifferentialComments

Summary:
Ref T2222. This adds PHIDs to all Differential comments so I can migrate the inlinecommment table to transaction_comment in the next diff.

@wez, this will issue a few million queries for Facebook (roughly, one for each Differential comment ever made). It's safe to skip the `.php` half of the patch, bring Phabricator up normally, and then apply this patch with Phabricator running if that eases the migration, although the next few diffs will probably be downtime…