Add an event for looking up names from repositories

Summary:
Currently, we have this cumbersome `PhabricatorRepositoryCommitMessageDetailParser` hook. This is really old and outdated; I want to just use the Differential custom field parser. See T945 for a specific application.

However, it allows installs to override author/committer association. Instead, provide an event hook for doing this.

Test Plan: Added a listener, made every commit resolve to "turtle", parsed some commits, verified the events looked sane and they now correctly were all attributed to "turtle".

Reviewers: btrahan, vrana, nh

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1337

Differential Revision: https://secure.phabricator.com/D3040

src/applications/repository/parser/PhabricatorRepositoryCommitMessageDetailParser.php

@@ -69,6 +69,10 @@ abstract class PhabricatorRepositoryCommitMessageDetailParser {
 
     $display_name = $email->getDisplayName();
     if ($display_name) {
+      $phid = $this->findUserByUserName($display_name);
+      if ($phid) {
+        return $phid;
+      }
       $phid = $this->findUserByRealName($display_name);
       if ($phid) {
         return $phid;