WIP: Add Git patch workflow

Attempts to fix #134.

This patch is still not optimal. Some examples of broken/missing features include:

- When Git patches are applied, the commit message and author are not imported correctly by `git am`. This is because of the initial "This patch is part of MXE" text, which `git am` doesn't know to ignore.
- If a patch doesn't apply for example because I first updates the tarball in recipe, the error is not handled gracefully. I want this target to be able to automatically rebase the old patches to newer releases.