Sync MaxGitDiffLineCharacters with conf/app.ini (#2779)

MAX_GIT_DIFF_LINE_CHARACTERS was updated in #1845 but the corresponding
default value of MaxGitDiffLineCharacters was not changed. This can lead
to inconsistencies.

modules/setting/setting.go

@@ -438,7 +438,7 @@ var (
 	}{
 		DisableDiffHighlight:     false,
 		MaxGitDiffLines:          1000,
-		MaxGitDiffLineCharacters: 500,
+		MaxGitDiffLineCharacters: 5000,
 		MaxGitDiffFiles:          100,
 		GCArgs:                   []string{},
 		Timeout: struct {