feat: parse lines based on line counts

[wolfv]

I started to work on this a little bit to make our parsing more robust against empty lines (e.g. when whitespace is removed) by counting the lines from the diff.

[wolfv]

I think this is also needed to fix #2 because right now a line like -- will be counted as a "line to be removed" while it's actually the signature marker.

[whitty]

What is the state of this changed? is it intended to be complete?

FYI: test failures on regression patches:

This is failing on the chunk header - but see below.

https://github.com/gitpatch-rs/gitpatch/actions/runs/18760185548/job/53522404788?pr=3#step:5:146

failed to parse "tests/samples/sample1.diff", error: Line 13: Error while parsing:  @@ -5,16 +11,13 @@
...

This patch seems to be being mis-parsed possibly around the end-of-chunk detection?

https://github.com/gitpatch-rs/gitpatch/actions/runs/18760185548/job/53522404788?pr=3#step:5:192

failed to parse "tests/wild-samples/sample2.patch", error: Line 27: Error while parsing: diff --git a/js/src/jsfriendapi.cpp b/js/src/jsfriendapi.cpp
....

[sourcefrog]

It seems like conceptually a good idea to make use of the counts in the headers, but I'm not really sure from the PR or a brief read of the diff which one is meant to have priority if the number of lines expected disagrees with the lines that are apparently present?

[wolfv]

Sorry for dropping the ball here. We've switched to our own fork of diffy and made it handle all edge cases we found in the wild: https://github.com/prefix-dev/diffy

IIRC there were some test cases here that did not contain valid diffs which made the line counting fail :) But I might misremember!