Improve Diffusion behavior for no-longer-existing commits

Summary:
Ref T9028. When users push a commit, then later delete it (e.g., by deleting the branch which contained it) we currently explode when trying to view it.

Instead, degrade gradually if some information is not available.

Test Plan:

• Looked at valid commits with parents, refs, branches and merges.
• Looked at invalid commits.
• Looked at a previously valid, now-deleted + gc'd commit:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9028

Differential Revision: https://secure.phabricator.com/D14227