diff_block: fix link to file revisions

Pull requests would link to file revisions in the wrong repo. That was
obviously only visible when merging between different repos - but then it would
link to a non-existing revision.

diff_block is apparently used pull-request-style with the 'b' revision of the
diff shown first. It thus also has to point at the 'other' repo which is where
the other revision can be found.