Updating a branch review where the base branch has been deleted causes the traversal of the entire repository

When updating (either manually or automatically), a branch review, where the base branch has been removed, the lookup to find commits on the reviewed branch, but excluding the base branch will consider every commit in the repository (up to the sanity limit of 50000).

If the updated branch does have some new revisions, looking up the start revisions for those (findStartRevisions) will consider every single ancestor of each revision, that is on one of the commits retrieved in the previous step.

The latter part might visit revisions multiple times for revisions with complex ancestries, and does an eager load of the revisions, potentially consuming a lot of memory, and causing the instance to die with an OutOfMemoryError.