commit: pass ChangingFiles object as argument to _process_files
Instead of returning it, we pass it as an argument. This makes the whole if-else in `_prepare_files` a bit simpler. Else each if-else branch was creating the object.
Instead of returning it, we pass it as an argument. This makes the whole if-else in `_prepare_files` a bit simpler. Else each if-else branch was creating the object.