Add migration to encourage rebuilding repository identities

Summary: Ref T12164. Defines a new manual activity that suggests rebuilding repository identities before Phabricator begins to rely on them.

Test Plan:
- Ran migration, observed expected setup issue: {F5788217}
- Ran `bin/config done identities` and observed setup issue get marked as done.
- Ran `/bin/storage upgrade --apply phabricator:20170912.ferret.01.activity.php` to make sure I didn't break the reindex migration; observed reindex setup issue appear as expected.
- Ran `./bin/config done reindex` and observed reindex issue cleared as expected.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T12164

Differential Revision: https://secure.phabricator.com/D19497

resources/sql/autopatches/20180809.repo_identities.activity.php

@@ -0,0 +1,20 @@
+<?php
+
+// Advise installs to rebuild the repository identities.
+
+// If the install has no commits (or no commits that lack an
+// authorIdentityPHID), don't require a rebuild.
+$commits = id(new PhabricatorRepositoryCommit())
+  ->loadAllWhere('authorIdentityPHID IS NULL LIMIT 1');
+
+if (!$commits) {
+  return;
+}
+
+try {
+  id(new PhabricatorConfigManualActivity())
+    ->setActivityType(PhabricatorConfigManualActivity::TYPE_IDENTITIES)
+    ->save();
+} catch (AphrontDuplicateKeyQueryException $ex) {
+  // If we've already noted that this activity is required, just move on.
+}