rc filetype diff: extract diff parsing from diff-jump

Most diff consumers we've written only care about the "final" state
after parsing through a diff. Let's extract the diff parsing part,
for reuse in several new commands.

In future we should try to use this (or better, a diff-parsing library)
for patch-range.pl. We'd add a callback argument that is invoked once
perl hunk (or line). Unfortunately I haven't found that library for
Perl yet.

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