compute: add diff result type
Created by: rvantonder
Add dedicated diff result type. It is currently only used/generated by compute. I implemented the result methods even though I don't need them, and they're not used yet, so that in time we should migrate to using this diff result type and not proxy via CommitMatch
.
Thanks @camdencheek for the diff parsing code in https://github.com/sourcegraph/sourcegraph/tree/cc/diff-parser!
Test plan
Added unit tests.