When importing Git repositories, treat out-of-range timestamps as the current time

Summary:
Fixes T11537. See that task for discussion.

Although we could accommodate these faithfully, it requires a huge migration and affects one repository on one install which was written with buggy tools.

At least for now, just replace out-of-32-bit-range epoch values with the current time, which is often somewhat close to the real value.

Test Plan:
  - Following the instructions in T11537, created commits in 40,000 AD.
  - Tried to import them, reproducing the "epoch" database issue.
  - Applied the patch.
  - Successfully imported future-commits, with some liberties around commit dates. Note that author date (not stored in an `epoch` column) is still shown faithfully:

{F1789302}

Reviewers: chad, avivey

Reviewed By: avivey

Maniphest Tasks: T11537

Differential Revision: https://secure.phabricator.com/D16456

src/applications/repository/engine/PhabricatorRepositoryDiscoveryEngine.php

@@ -406,9 +406,17 @@ final class PhabricatorRepositoryDiscoveryEngine
 
     $refs = array();
     foreach ($commits as $commit) {
+      $epoch = $stream->getCommitDate($commit);
+
+      // If the epoch doesn't fit into a uint32, treat it as though it stores
+      // the current time. For discussion, see T11537.
+      if ($epoch > 0xFFFFFFFF) {
+        $epoch = PhabricatorTime::getNow();
+      }
+
       $refs[] = id(new PhabricatorRepositoryCommitRef())
         ->setIdentifier($commit)
-        ->setEpoch($stream->getCommitDate($commit))
+        ->setEpoch($epoch)
         ->setCanCloseImmediately($close_immediately)
         ->setParents($stream->getParents($commit));
     }