Add more tests for diff highlighting

[mrsdizzie]

Adding some more tests for different cases to make sure they keep working

[codecov-commenter]

Codecov Report

Merging #12467 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12467      +/-   ##
==========================================
- Coverage   43.80%   43.76%   -0.04%     
==========================================
  Files         630      630              
  Lines       69821    69821              
==========================================
- Hits        30583    30558      -25     
- Misses      34299    34318      +19     
- Partials     4939     4945       +6     

Impacted Files	Coverage Δ
services/gitdiff/gitdiff.go	73.88% <ø> (+0.66%)	⬆️
services/pull/temp_repo.go	26.59% <0.00%> (-6.39%)	⬇️
modules/sync/unique_queue.go	38.77% <0.00%> (-6.13%)	⬇️
services/pull/update.go	51.92% <0.00%> (-5.77%)	⬇️
modules/git/repo_language_stats.go	52.83% <0.00%> (-3.78%)	⬇️
services/pull/check.go	50.78% <0.00%> (-3.13%)	⬇️
services/pull/patch.go	66.37% <0.00%> (-2.59%)	⬇️
modules/git/repo.go	49.74% <0.00%> (-2.04%)	⬇️
modules/log/event.go	57.54% <0.00%> (-0.95%)	⬇️
services/pull/pull.go	42.03% <0.00%> (-0.24%)	⬇️
... and 2 more

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