This change adds a server endpoint to diff two git refs for a single file. This is flexible to perform diffs across various points in the Git lifecycle such as diff'ing changes in the Working Tree, Index, or two arbitrary commits. There are two reserverd refs WORKING and INDEX to designate the Working Tree and the git Index

Additionally,

* Improve error handling to return a JSON response so that the frontend can process it gracefully.

Again, this is backwards compatible with the existing /diff API which uses the "git:" prefixes.

Avoid extra param and other changes to internal functions, whie still supporting the new cleaner API.

If a file was added or deleted, GitPython will indicate the blob as None. In the current case, we would always read the file from disk so the file contents would be same as the Working Tree. This commit modifies that handling to handle the Working Tree case separately, and the None blob case separately.

Tested on the both the nbdime and Git lab extensions.