R1:1459fb303769

Make re-running `rebuild-identities` a bit faster and add a little progress information

Summary:
Ref T13151. Ref T12164. Two small tweaks:

- If we aren't actually going to change anything, just skip the writes. This makes re-running/resuming a lot faster (~20x, locally).
- Print when we touch a commit so there's some kind of visible status.

This is just a small quality-of-life tweak that I wrote anyway while investigating T13152, and will make finishing off db024, db025 and db010 manually a little…