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

Given the following markdown:

```
a832c723cd
19fe6cdf81
a832c723cd
```

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_commit.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/modules/base"
+	"code.gitea.io/gitea/modules/httplib"
 	"code.gitea.io/gitea/modules/references"
 	"code.gitea.io/gitea/modules/util"
 
@@ -121,6 +122,11 @@ func fullHashPatternProcessor(ctx *RenderContext, node *html.Node) {
 		if ret.QueryHash != "" {
 			text += " (" + ret.QueryHash + ")"
 		}
+		// only turn commit links to the current instance into hash link
+		if !httplib.IsCurrentGiteaSiteURL(ctx, ret.FullURL) {
+			node = node.NextSibling
+			continue
+		}
 		replaceContent(node, ret.PosStart, ret.PosEnd, createCodeLink(ret.FullURL, text, "commit"))
 		node = node.NextSibling.NextSibling
 	}
@@ -167,6 +173,12 @@ func comparePatternProcessor(ctx *RenderContext, node *html.Node) {
 			}
 		}
 
+		// only turn compare links to the current instance into hash link
+		if !httplib.IsCurrentGiteaSiteURL(ctx, urlFull) {
+			node = node.NextSibling
+			continue
+		}
+
 		text := text1 + textDots + text2
 		if hash != "" {
 			text += " (" + hash + ")"