For twig we had the problem that the basic patch was not accepted, but that the followup issues should be clean.
The idea is to use two patches within one.
What do you do if you happen to find a patch with two patches in one?
For patch contributors the workflow is not that difficult to the regular patches:
Assuming origin is the "core" repository and you work in feature branches the workflow is:
Lets further assume the patch is in issue 1806546 with comment number 4
Getting a clean branch:
# update core
git fetch origin
# checkout 8.x branch
git checkout origin/8.x
# checking out an issue feature branch with comment number
git checkout -b core-1806546-4
Applying the current patch:
# this directly _commits_ the patches, conflicts are resolved like
# with rebase / merge
git am patch.diff
# Now you fork again with your new comment number
git checkout -b core-1806546-5
Now you add your changes and then you do:
# this updates the original patch and can be done as often as you want
git commit --amend .
# this creates the new 2-part patch
git format-patch -M --stdout origin/8.x > mynewpatch.diff
# this creates the interdiff
git format-patch -M --stdout core-1806546-4 > interdiff.txt