Refactor markup render system (#32612)

This PR removes (almost) all path tricks, and introduces "renderhelper"
package.

Now we can clearly see the rendering behaviors for comment/file/wiki,
more details are in "renderhelper" tests.

Fix #31411 , fix #18592, fix #25632 and maybe more problems. (ps: fix
#32608 by the way)

models/renderhelper/repo_comment.go

@@ -0,0 +1,73 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package renderhelper
+
+import (
+	"context"
+	"fmt"
+
+	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/util"
+)
+
+type RepoComment struct {
+	ctx  *markup.RenderContext
+	opts RepoCommentOptions
+
+	commitChecker *commitChecker
+	repoLink      string
+}
+
+func (r *RepoComment) CleanUp() {
+	_ = r.commitChecker.Close()
+}
+
+func (r *RepoComment) IsCommitIDExisting(commitID string) bool {
+	return r.commitChecker.IsCommitIDExisting(commitID)
+}
+
+func (r *RepoComment) ResolveLink(link string, likeType markup.LinkType) (finalLink string) {
+	switch likeType {
+	case markup.LinkTypeApp:
+		finalLink = r.ctx.ResolveLinkApp(link)
+	default:
+		finalLink = r.ctx.ResolveLinkRelative(r.repoLink, r.opts.CurrentRefPath, link)
+	}
+	return finalLink
+}
+
+var _ markup.RenderHelper = (*RepoComment)(nil)
+
+type RepoCommentOptions struct {
+	DeprecatedRepoName  string // it is only a patch for the non-standard "markup" api
+	DeprecatedOwnerName string // it is only a patch for the non-standard "markup" api
+	CurrentRefPath      string // eg: "branch/main" or "commit/11223344"
+}
+
+func NewRenderContextRepoComment(ctx context.Context, repo *repo_model.Repository, opts ...RepoCommentOptions) *markup.RenderContext {
+	helper := &RepoComment{
+		repoLink: repo.Link(),
+		opts:     util.OptionalArg(opts),
+	}
+	rctx := markup.NewRenderContext(ctx)
+	helper.ctx = rctx
+	if repo != nil {
+		helper.repoLink = repo.Link()
+		helper.commitChecker = newCommitChecker(ctx, repo)
+		rctx = rctx.WithMetas(repo.ComposeMetas(ctx))
+	} else {
+		// this is almost dead code, only to pass the incorrect tests
+		helper.repoLink = fmt.Sprintf("%s/%s", helper.opts.DeprecatedOwnerName, helper.opts.DeprecatedRepoName)
+		rctx = rctx.WithMetas(map[string]string{
+			"user": helper.opts.DeprecatedOwnerName,
+			"repo": helper.opts.DeprecatedRepoName,
+
+			"markdownLineBreakStyle":       "comment",
+			"markupAllowShortIssuePattern": "true",
+		})
+	}
+	rctx = rctx.WithHelper(helper)
+	return rctx
+}