Only turn links to current instance into hash links (#36237)

Given the following markdown:

```
http://localhost:3500/silverwind/symlink-test/commit/a832c723cd116df44cce6271c4a89afa4d8ec670
http://localhost:3500/silverwind/remap-css/commit/19fe6cdf81f7ec50b8cac2d6c28fe7c42c1ffe14
http://github.com/silverwind/symlink-test/commit/a832c723cd116df44cce6271c4a89afa4d8ec670
```

Previously, all links would turn into hash link, even ones to external
sites:

<img width="849" height="89" alt="Screenshot 2025-12-23 at 19 19 13"
src="https://github.com/user-attachments/assets/2ad35a18-4542-40a4-a838-7ab8ac8bc047"
/>

After this change, only links to the current instance, as identified by
`setting.AppURL` are turned into hash links:

<img width="850" height="87" alt="Screenshot 2025-12-23 at 19 18 56"
src="https://github.com/user-attachments/assets/2c49a5b2-426c-4a82-a610-9b9da8dcfff9"
/>

There is still one notable [difference with
GitHub](https://github.com/silverwind/symlink-test/issues/20#issuecomment-3687535938)
where the second link should render like `user/repo@<hash>`, not
`<hash>` as currently, I would like to fix that here as well.

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

modules/markup/html_internal_test.go

@@ -299,7 +299,7 @@ func TestRender_AutoLink(t *testing.T) {
 
 	// render other commit URLs
 	tmp = "https://external-link.gitea.io/go-gitea/gitea/commit/d8a994ef243349f321568f9e36d5c3f444b99cae#diff-2"
-	test(tmp, "<a href=\""+tmp+"\" class=\"commit\"><code>d8a994ef24 (diff-2)</code></a>")
+	test(tmp, "<a href=\""+tmp+"\">"+tmp+"</a>")
 }
 
 func TestRender_FullIssueURLs(t *testing.T) {