typing: add some type annotations to the `merge.mergeresult` class The generated type annotations around `filemap()` and `files()` were slightly modified by the pyupgrade series culminating in 70a75d379daf. This module is way more complicated than the other changes, but these weren't too bad to figure out. The typing caught a trivial issue in `sparse`- it was passing an empty data list to `addfile()` for the `ACTION_REMOVE` case, instead of a tuple or None. `merge.manifestmerge()` calls this function with None for the data, so 1) it has to be typed as optional, and 2) is safe to pass None in the sparse code.