Add an "Unreachable" flag for commits and revive them during discovery

Summary:
Ref T9028. This is the easy part of dealing with deleted commits:

  - Add a flag for unreachable commits (nothing sets this flag yet).
  - Ignore unreachable commits when querying for known commits during discovery, so we pretend they do not exist.
  - When recording a commit, try just reviving an existing unreachable commit first. If that works, bail out.

Test Plan:
  - Artificially marked a commit as unreachable with raw SQL.
  - Verified it said "deleted: unreachable" in the UI.
  - Ran `repository discover --trace --verbose`.
  - Saw the discovery process ignore the commit when filling the cache.
  - Saw the discovery process revive the commit instead of trying to record it again.
  - Web UI now shows the commit as normal.
  - Running `repository discover` again doesn't make any further changes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9028

Differential Revision: https://secure.phabricator.com/D16130

src/applications/diffusion/controller/DiffusionCommitController.php

@@ -129,6 +129,12 @@ final class DiffusionCommitController extends DiffusionController {
           ),
           $message));
 
+      if ($commit->isUnreachable()) {
+        $this->commitErrors[] = pht(
+          'This commit has been deleted in the repository: it is no longer '.
+          'reachable from any branch, tag, or ref.');
+      }
+
       if ($this->getCommitErrors()) {
         $error_panel = id(new PHUIInfoView())
           ->appendChild($this->getCommitErrors())