Git diff request correctly throws NotFoundException if target revision is non-existent

[Pilopa]

Proposed changes

Sometimes it happens that a git diff command request is performed with a non-existent target branch. This is usually fine but the underlying system might have already garbage-collected the revisions associated with that branch. In this case, the revision for that deleted branch might turn up null which currently causes a 500 error. We catch this specific corner-case and throw the correct NotFoundException instead.

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