Full-file syntax highlighting for diff pages (#33766)

Fix #33358, fix #21970

This adds a step in the `GitDiffForRender` that does syntax highlighting for the
entire file and then only references lines from that syntax highlighted
code. This allows things like multi-line comments to be syntax
highlighted correctly.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/api/v1/repo/pull.go

@@ -1591,8 +1591,7 @@ func GetPullRequestFiles(ctx *context.APIContext) {
 	maxLines := setting.Git.MaxGitDiffLines
 
 	// FIXME: If there are too many files in the repo, may cause some unpredictable issues.
-	// FIXME: it doesn't need to call "GetDiff" to do various parsing and highlighting
-	diff, err := gitdiff.GetDiff(ctx, baseGitRepo,
+	diff, err := gitdiff.GetDiffForAPI(ctx, baseGitRepo,
 		&gitdiff.DiffOptions{
 			BeforeCommitID:     startCommitID,
 			AfterCommitID:      endCommitID,