Save git branches of patch submissions

We could integrate patchwork with git a bit more intimately. Whenever a series is sent we could try to apply the series to a branch and store it somewhere in the an instance of the project git repo. This way we could:
- ensure submissions do apply, send a mail back otherwise
- have a branch with the patches applied so git am can use 3-way merges with the info of that branch
- check if work in flight are conflicting with each other (a background task that would try to merge each branch).

This doesn't have to be part of patchwork itself but could be done externally, just like the CI systems work today