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 This is really old and outdated; I want to just use the Differential custom field parser.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/docs/userguide/events.diviner

@@ -138,6 +138,35 @@ will be available yet. Data available on this event:
   - `repository` The @{class:PhabricatorRepository} the commit was discovered
     in.
 
+== Diffusion: Lookup User ==
+
+The constant for this event is
+`PhabricatorEventType::TYPE_DIFFUSION_LOOKUPUSER`.
+
+This event is dispatched when the daemons are trying to link a commit to a
+Phabricator user account. You can listen for it to improve the accuracy of
+associating users with their commits.
+
+By default, Phabricator will try to find matches based on usernames, real names,
+or email addresses, but this can result in incorrect matches (e.g., if you have
+several employees with the same name) or failures to match (e.g., if someone
+changed their email address). Listening for this event allows you to intercept
+the lookup and supplement the results from another datasource.
+
+Data available on this event:
+
+  - `commit` The @{class:PhabricatorRepositoryCommit} that data is being looked
+    up for.
+  - `query` The author or committer string being looked up. This will usually
+    be something like "Abraham Lincoln <alincoln@logcabin.example.com>", but
+    comes from the commit metadata so it may not be well-formatted.
+  - `result` The current result from the lookup (Phabricator's best guess at
+    the user PHID of the user named in the "query"). To substitute the result
+    with a different result, replace this with the correct PHID in your event
+    listener.
+
+Using @{class:PhutilEmailAddress} may be helpful in parsing the query.
+
 == Edge: Will Edit Edges ==
 
 NOTE: Edge events are low-level events deep in the core. It is more difficult to