extdiff: remove unrequired do3way argument to diffpatch()
When we are diffing patches, there is no 3 way diff.
Differential Revision: https://phab.mercurial-scm.org/D8688
extdiff: refactor logic to diff revs of versions of files
Now that code for both cases, diffing patches or files is in separate function,
it will be better to refactor them more and understand.
Differential Revision: https://phab.mercurial-scm.org/D8687