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/web/repo/blame.go

@@ -97,7 +97,7 @@ func RefBlame(ctx *context.Context) {
 		return
 	}
 
-	ctx.Data["NumLines"], err = blob.GetBlobLineCount()
+	ctx.Data["NumLines"], err = blob.GetBlobLineCount(nil)
 	if err != nil {
 		ctx.NotFound(err)
 		return

routers/web/repo/commit.go

@@ -314,7 +314,7 @@ func Diff(ctx *context.Context) {
 		maxLines, maxFiles = -1, -1
 	}
 
-	diff, err := gitdiff.GetDiff(ctx, gitRepo, &gitdiff.DiffOptions{
+	diff, err := gitdiff.GetDiffForRender(ctx, gitRepo, &gitdiff.DiffOptions{
 		AfterCommitID:      commitID,
 		SkipTo:             ctx.FormString("skip-to"),
 		MaxLines:           maxLines,

routers/web/repo/compare.go

@@ -614,7 +614,7 @@ func PrepareCompareDiff(
 
 	fileOnly := ctx.FormBool("file-only")
 
-	diff, err := gitdiff.GetDiff(ctx, ci.HeadGitRepo,
+	diff, err := gitdiff.GetDiffForRender(ctx, ci.HeadGitRepo,
 		&gitdiff.DiffOptions{
 			BeforeCommitID:     beforeCommitID,
 			AfterCommitID:      headCommitID,

routers/web/repo/pull.go

@@ -749,7 +749,7 @@ func viewPullFiles(ctx *context.Context, specifiedStartCommit, specifiedEndCommi
 		diffOptions.BeforeCommitID = startCommitID
 	}
 
-	diff, err := gitdiff.GetDiff(ctx, gitRepo, diffOptions, files...)
+	diff, err := gitdiff.GetDiffForRender(ctx, gitRepo, diffOptions, files...)
 	if err != nil {
 		ctx.ServerError("GetDiff", err)
 		return