Simplify split diff view generation and remove JS dependency (#16775) (#16863)

Backport #16775

Gitea has relied on some slow JS code to match up added and deleted lines on the
diff pages. This can cause a considerable slow down on large diff pages.

This PR makes a small change meaning that the matching up can occur much more simply.

Partial fix #1351

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/repofiles/diff_test.go

@@ -83,6 +83,7 @@ func TestGetDiffPreview(t *testing.T) {
 							{
 								LeftIdx:  3,
 								RightIdx: 0,
+								Match:    4,
 								Type:     3,
 								Content:  "-Description for repo1",
 								Comments: nil,
@@ -90,6 +91,7 @@ func TestGetDiffPreview(t *testing.T) {
 							{
 								LeftIdx:  0,
 								RightIdx: 3,
+								Match:    3,
 								Type:     2,
 								Content:  "+Description for repo1",
 								Comments: nil,
@@ -97,6 +99,7 @@ func TestGetDiffPreview(t *testing.T) {
 							{
 								LeftIdx:  0,
 								RightIdx: 4,
+								Match:    -1,
 								Type:     2,
 								Content:  "+this is a new line",
 								Comments: nil,