Pass repository PHID to custom hooks in PHABRICATOR_REPOSITORY instead of callsign

Summary:
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.

Test Plan:
  - 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`:

```name=stuff.sh
#!/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
```

Reviewers: chad, avivey

Reviewed By: avivey

Subscribers: avivey

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D15294

src/applications/diffusion/engine/DiffusionCommitHookEngine.php

@@ -10,6 +10,7 @@
  */
 final class DiffusionCommitHookEngine extends Phobject {
 
+  const ENV_REPOSITORY = 'PHABRICATOR_REPOSITORY';
   const ENV_USER = 'PHABRICATOR_USER';
   const ENV_REMOTE_ADDRESS = 'PHABRICATOR_REMOTE_ADDRESS';
   const ENV_REMOTE_PROTOCOL = 'PHABRICATOR_REMOTE_PROTOCOL';
@@ -610,7 +611,7 @@ final class DiffusionCommitHookEngine extends Phobject {
     $console = PhutilConsole::getConsole();
 
     $env = array(
-      'PHABRICATOR_REPOSITORY' => $this->getRepository()->getCallsign(),
+      self::ENV_REPOSITORY => $this->getRepository()->getPHID(),
       self::ENV_USER => $this->getViewer()->getUsername(),
       self::ENV_REMOTE_PROTOCOL => $this->getRemoteProtocol(),
       self::ENV_REMOTE_ADDRESS => $this->getRemoteAddress(),

src/docs/user/userguide/diffusion_hooks.diviner

@@ -42,7 +42,7 @@ These hooks act like normal `pre-commit` or `pre-receive` hooks:
 These additional variables will be available in the environment, in addition
 to the variables the VCS normally provides:
 
-  - `PHABRICATOR_REPOSITORY` The callsign of the repository the hook is
+  - `PHABRICATOR_REPOSITORY` The PHID of the repository the hook is
     executing for.
   - `PHABRICATOR_USER` The Phabricator username that the session is
     authenticated under.