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.

• Ugh.
• Picked a hosted Git repo out of Diffusion.
• Went to the path on disk.
• Went into hooks/.
• Went into pre-receive-phabricator.d/.
• Wrote this hook and gave it chmod +x:

#!/bin/sh

echo $PHABRICATOR_REPOSITORY >> /tmp/stuff.log

• Pushed to the repository.
• Saw a PHID show up in the log:

$ cat /tmp/stuff.log
PHID-REPO-bqkcdp47euwnwlasrsrh