Survive hand-crafted Git commits which are missing timestamp information

Summary:
Fixes T12062. Like the commits from the year 3500, you can artificially build commits with no date information.

We could explicitly store these as `null` to fully respect the underlying datastore. However, I think it's very unlikely that these commits are intentional/meaningful or that this is valuable.

Additionally, "git show" interprets these commits as "Jan 1, 1970". Just store a `0` to mimic its behavior.

Test Plan:
  - Following the process in T11537#192019, artificially created a commit with //no// date information (I deleted all date information from the message).
  - Used `git show` / `git log --format ...` to inspect it: "Jan 1, 1970" on `git show`, no information at all on `%aD`, `%aT`, etc.
  - Pushed it.
  - Saw exception for trying to insert empty string into epoch colum from `bin/repository update`.
  - Applied patch.
  - Got a clean import.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12062

Differential Revision: https://secure.phabricator.com/D17136

src/applications/repository/engine/PhabricatorRepositoryDiscoveryEngine.php

@@ -414,6 +414,13 @@ final class PhabricatorRepositoryDiscoveryEngine
         $epoch = PhabricatorTime::getNow();
       }
 
+      // If the epoch is not present at all, treat it as though it stores the
+      // value "0". For discussion, see T12062. This behavior is consistent
+      // with the behavior of "git show".
+      if (!strlen($epoch)) {
+        $epoch = 0;
+      }
+
       $refs[] = id(new PhabricatorRepositoryCommitRef())
         ->setIdentifier($commit)
         ->setEpoch($epoch)