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