diff: move b85diff to mdiff module
b85diff generates a binary diff, so we move this code to mdiff module
along with unidiff for text diffs. All diffing mechanisms will be in the
same place.
In an upcoming patch we will remove the responsibility to print the
index header from b85diff and move it back to patch, since it's
a patch metadata header, not part of the diff generation.
basic:
creating
creating
creating
creating
fakeuncacheable:
creating
creating
creating
creating
creating
creating