Showing a diff between two versions of the same series #636

Hello,

First, thank you for developing and maintaining this great tool!

For the MPTCP project, we are also using Patchew, mainly to apply patches in a Git repo, but also for extra features like showing a diff between two versions of the same series. Example: when looking at a series on Patchew, you can see at the top "Diff against v1 v2 v3"

Showing the diff between versions only need access to the patches, no need to apply them in a repo. Here is what Patchew is doing with the help of jsdifflib: diff module & diff template. Ideally, patchwork would need the ability to track series' versions (which would be great for the reviewers to quickly jump between versions), but this can also be optional: the first version could start with only showing a diff between two series' message-ids given by the user for example.

(Note that having Patchwork being able to directly apply patches in a repo would be great, not depends on another service regularly polling for new events, but that's another topic :) )