Don't try to rebuild a repository graph cache bucket more than once per request

Summary: Ref T2683. This is a small optimization, but it has low complexity: don't rebuild a bucket more than once in the same request, since it will almost always be the same. Bucket rebuilds are pretty cheap, but this saves a few queries.

Test Plan:

• After discovering (but before parsing) a commit, viewed its browse view. Verified that this patch causes us to perform only one bucket rebuild, and therefore reduces the number of queries we issue.
• Parsed the commit and viewed the browse view again, got successful rebuild and then fills from cache.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9055