Add a lineReader to provide two-line lookahead

[mrnugget]

No description provided.

[eseliger]

Cool, makes sense to me to look two lines ahead.

[codecov]

Codecov Report

Merging #56 into master will increase coverage by 0.49%.
The diff coverage is 90.32%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   74.10%   74.59%   +0.49%     
==========================================
  Files           4        4              
  Lines         421      437      +16     
==========================================
+ Hits          312      326      +14     
- Misses         62       63       +1     
- Partials       47       48       +1     

Impacted Files	Coverage Δ
diff/reader_util.go	84.84% <85.00%> (+0.23%)	⬆️
diff/parse.go	81.25% <100.00%> (+0.08%)	⬆️

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[keegancsmith]

I still feel a bit iffy about error conditions here, but its a clear improvement over previous code. Great thing is we have a great test case that is sourcegraph.com, so in practice this is fine :D

[mrnugget]

Great thing is we have a great test case that is sourcegraph.com, so in practice this is fine :D

That's the spirit!

I want to add more tests to this PR to check for more conditions. I also think it makes sense to add a more integration-level test here that makes sure that a huge diff can be parsed correctly. We have unit tests for the subparts, but I think a test that checks for multi-file diffs, including hunk contents, is missing.