Improve diff highlight (#3390)

- Try to reduce memory allocations
- Add possibility to disable diff highlight (can improve performance for large diffs)
- Tweaking with cost for prettier (cleaner) diffs
- Do not calculate diff when the number of removed lines in a block is not equal to the number of added lines (this usually resulted in ugly diffs)

modules/setting/setting.go

@@ -191,6 +191,7 @@ var (
 
 	// Git settings
 	Git struct {
+		DisableDiffHighlight     bool
 		MaxGitDiffLines          int
 		MaxGitDiffLineCharacters int
 		MaxGitDiffFiles          int