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
extdiff: refactor logic which does diff of patches
The current extdiff code is hard to understand on first look. Took me few hours
to grasp the code. Before adding more things, decided to do some refactoring.
Differential Revision: https://phab.mercurial-scm.org/D8686