Blob.context() (which is called when on the file view page) calls
Blob.next_commit but the result is not used.
Blob.next_commit walks down the commit tree to find the next commit that modified the file, which for rarely modified files can result in a lot of useless work.
Blob.prev_commit is more efficient, but is also not used.
Remove both, as well as associated tests in
Allura/allura/tests/unit/test_repo.py, and ensure there is no visible impact to the UI of the file view page.