Fix a fatal in blame when the viewer can't see a revision because of a permission issue

Summary:
Fixes T13040. To reproduce:

  - View a file with blame enabled, where some line has an associated revision (say, `D123`).
  - Edit `D123` so it exists and is a valid revision, but the viewer can't see it.
  - Reload the page.

Instead, only add revisions to the map if we actually managed to load them.

Test Plan: Page no longer fatals.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13040

Differential Revision: https://secure.phabricator.com/D18884

src/applications/diffusion/controller/DiffusionBrowseController.php

@@ -947,6 +947,10 @@ final class DiffusionBrowseController extends DiffusionController {
       }
 
       foreach ($revision_ids as $commit_phid => $revision_id) {
+        // If the viewer can't actually see this revision, skip it.
+        if (!isset($revisions[$revision_id])) {
+          continue;
+        }
         $revision_map[$commit_map[$commit_phid]] = $revision_id;
       }
     }