Ref T4245. We pass this exclusively for use by additional third-party hooks.
This is technically a backward compatibility break, but I suspect it doesn't affect anyone:
- Probably almost no one is using this (there are few reasons to, even for the tiny number of installs with custom commit hooks).
- If they are, there's a good chance the PHID will work anyway, since nearly all scripts and Conduit methods will accept it in place of a callsign now, and if it's in logging or debugging code the PHID is a reasonable substitute
- Even if it doesn't just keep working, the break should be very obvious in most reasonable cases.
I'll call this out explicitly in the changelog, though -- almost everything else will just continue working, but this is a strict compatibility break.