Support renamed binary files without index-row

[zegl]

I found a wild diff generated by libgit2 that was previously unparsable by go-diff. The only difference in this diff (included as testdata) and diffs that are parsable by go-diff is the "index" row, such as "index 17a971d..599f8dd 100644".

This commit adds support for renamed binary files without that index row.

[codecov]

Codecov Report

Merging #63 (c26dd7a) into master (35b24a7) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   77.34%   77.39%   +0.04%     
==========================================
  Files           4        4              
  Lines         490      491       +1     
==========================================
+ Hits          379      380       +1     
  Misses         63       63              
  Partials       48       48              

Impacted Files	Coverage Δ
diff/parse.go	84.07% <100.00%> (+0.04%)	⬆️

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[mrnugget]

Thanks! Change looks good to me, although I wonder: under which circumstances is a diff like that produced?