revert: detect files added during a merge In case of merge, file that are actually added can be reported as modified. This is currently handled by special-case code. We detect it beforehand instead. This will lets use remove the special-case code at some point in the future.

revert: simplify handling of `added` files There are multiple possible cases for added files. But it's all handled by magic much lower in the stack. We document them, simplify the codes and move on.