Allows restricting patches to a specific subdirectory, à la
`git diff --relative=subdir`.
This cannot be done (cleanly) currently because the `includes` logic
happens *after* `stripLen` is applied, so we can't match on `subdir/*`.
This change adds a `relative` argument that makes this possible by
filtering files before doing any processing, and setting `stripLen` and
`extraPrefix` accordingly.
Naïm Favier
bf7d13dc4f