Use full-file highlighting for diff sections (#36561)

* Fix #35252
* Fix #35999
* Improve diff rendering, don't add unnecessary "added"/"removed" tags for a full-line change
* Also fix a "space trimming" bug in #36539 and add tests
* Use chroma "SQL" lexer instead of "MySQL" to workaround a bug (35999)

routers/web/repo/editor_preview.go

@@ -6,12 +6,13 @@ package repo
 import (
 	"net/http"
 
+	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/services/context"
 	files_service "code.gitea.io/gitea/services/repository/files"
 )
 
 func DiffPreviewPost(ctx *context.Context) {
-	content := ctx.FormString("content")
+	newContent := ctx.FormString("content")
 	treePath := files_service.CleanGitTreePath(ctx.Repo.TreePath)
 	if treePath == "" {
 		ctx.HTTPError(http.StatusBadRequest, "file name to diff is invalid")
@@ -27,7 +28,12 @@ func DiffPreviewPost(ctx *context.Context) {
 		return
 	}
 
-	diff, err := files_service.GetDiffPreview(ctx, ctx.Repo.Repository, ctx.Repo.BranchName, treePath, content)
+	oldContent, err := entry.Blob().GetBlobContent(setting.UI.MaxDisplayFileSize)
+	if err != nil {
+		ctx.ServerError("GetBlobContent", err)
+		return
+	}
+	diff, err := files_service.GetDiffPreview(ctx, ctx.Repo.Repository, ctx.Repo.BranchName, treePath, oldContent, newContent)
 	if err != nil {
 		ctx.ServerError("GetDiffPreview", err)
 		return