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_test.go

@@ -0,0 +1,76 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package renderhelper
+
+import (
+	"context"
+	"testing"
+
+	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/markup/markdown"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRepoComment(t *testing.T) {
+	unittest.PrepareTestEnv(t)
+
+	repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+
+	t.Run("AutoLink", func(t *testing.T) {
+		rctx := NewRenderContextRepoComment(context.Background(), repo1).WithMarkupType(markdown.MarkupName)
+		rendered, err := markup.RenderString(rctx, `
+65f1bf27bc3bf70f64657658635e66094edbcb4d
+#1
+@user2
+`)
+		assert.NoError(t, err)
+		assert.Equal(t,
+			`<p><a href="/user2/repo1/commit/65f1bf27bc3bf70f64657658635e66094edbcb4d" rel="nofollow"><code>65f1bf27bc</code></a><br/>
+<a href="/user2/repo1/issues/1" class="ref-issue" rel="nofollow">#1</a><br/>
+<a href="/user2" rel="nofollow">@user2</a></p>
+`, rendered)
+	})
+
+	t.Run("AbsoluteAndRelative", func(t *testing.T) {
+		rctx := NewRenderContextRepoComment(context.Background(), repo1).WithMarkupType(markdown.MarkupName)
+
+		// It is Gitea's old behavior, the relative path is resolved to the repo path
+		// It is different from GitHub, GitHub resolves relative links to current page's path
+		rendered, err := markup.RenderString(rctx, `
+[/test](/test)
+[./test](./test)
+![/image](/image)
+![./image](./image)
+`)
+		assert.NoError(t, err)
+		assert.Equal(t,
+			`<p><a href="/user2/repo1/test" rel="nofollow">/test</a><br/>
+<a href="/user2/repo1/test" rel="nofollow">./test</a><br/>
+<a href="/user2/repo1/image" target="_blank" rel="nofollow noopener"><img src="/user2/repo1/image" alt="/image"/></a><br/>
+<a href="/user2/repo1/image" target="_blank" rel="nofollow noopener"><img src="/user2/repo1/image" alt="./image"/></a></p>
+`, rendered)
+	})
+
+	t.Run("WithCurrentRefPath", func(t *testing.T) {
+		rctx := NewRenderContextRepoComment(context.Background(), repo1, RepoCommentOptions{CurrentRefPath: "/commit/1234"}).
+			WithMarkupType(markdown.MarkupName)
+
+		// the ref path is only used to render commit message: a commit message is rendered at the commit page with its commit ID path
+		rendered, err := markup.RenderString(rctx, `
+[/test](/test)
+[./test](./test)
+![/image](/image)
+![./image](./image)
+`)
+		assert.NoError(t, err)
+		assert.Equal(t, `<p><a href="/user2/repo1/test" rel="nofollow">/test</a><br/>
+<a href="/user2/repo1/commit/1234/test" rel="nofollow">./test</a><br/>
+<a href="/user2/repo1/image" target="_blank" rel="nofollow noopener"><img src="/user2/repo1/image" alt="/image"/></a><br/>
+<a href="/user2/repo1/commit/1234/image" target="_blank" rel="nofollow noopener"><img src="/user2/repo1/commit/1234/image" alt="./image"/></a></p>
+`, rendered)
+	})
+}