Simplify template helper functions (#24570)

To avoid bloating the template helper functions, some functions could be
provided by type methods.

And the new code `data-line-type="{{.GetHTMLDiffLineType}}"` reads
better than `data-line-type="{{DiffLineTypeToStr .GetType}}"`


After the fix, screenshots (the same as before):

<details>


![image](https://user-images.githubusercontent.com/2114189/236657918-20ce01e0-1192-443e-aeb4-6b3fe1aa2102.png)


![image](https://user-images.githubusercontent.com/2114189/236657950-ee19727f-a1fc-4133-afc7-e5d1a8c1783f.png)

</details>

services/gitdiff/gitdiff.go

@@ -104,6 +104,19 @@ func (d *DiffLine) GetType() int {
 	return int(d.Type)
 }
 
+// GetHTMLDiffLineType returns the diff line type name for HTML
+func (d *DiffLine) GetHTMLDiffLineType() string {
+	switch d.Type {
+	case DiffLineAdd:
+		return "add"
+	case DiffLineDel:
+		return "del"
+	case DiffLineSection:
+		return "tag"
+	}
+	return "same"
+}
+
 // CanComment returns whether a line can get commented
 func (d *DiffLine) CanComment() bool {
 	return len(d.Comments) == 0 && d.Type != DiffLineSection