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)

modules/markup/render_link_test.go

@@ -0,0 +1,27 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markup
+
+import (
+	"context"
+	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestResolveLinkRelative(t *testing.T) {
+	ctx := context.Background()
+	setting.AppURL = "http://localhost:3000"
+	assert.Equal(t, "/a", resolveLinkRelative(ctx, "/a", "", "", false))
+	assert.Equal(t, "/a/b", resolveLinkRelative(ctx, "/a", "b", "", false))
+	assert.Equal(t, "/a/b/c", resolveLinkRelative(ctx, "/a", "b", "c", false))
+	assert.Equal(t, "/a/c", resolveLinkRelative(ctx, "/a", "b", "/c", false))
+	assert.Equal(t, "http://localhost:3000/a", resolveLinkRelative(ctx, "/a", "", "", true))
+
+	// some users might have used absolute paths a lot, so if the prefix overlaps and has enough slashes, we should tolerate it
+	assert.Equal(t, "/owner/repo/foo/owner/repo/foo/bar/xxx", resolveLinkRelative(ctx, "/owner/repo/foo", "", "/owner/repo/foo/bar/xxx", false))
+	assert.Equal(t, "/owner/repo/foo/bar/xxx", resolveLinkRelative(ctx, "/owner/repo/foo/bar", "", "/owner/repo/foo/bar/xxx", false))
+}