improve the compare page (#36261)

- The compare page head title should be `compare` but not `new pull
request`.
- Use `UnstableGuessRefByShortName` instead of duplicated functions
calls.
- Direct-compare, tags, commits compare will not display `New Pull
Request` button any more.

The new screenshot
<img width="1459" height="391" alt="image"
src="https://github.com/user-attachments/assets/64e9b070-9c0b-41d1-b4b8-233b96270e1b"
/>

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/ref.go

@@ -220,3 +220,14 @@ func (ref RefName) RefWebLinkPath() string {
 	}
 	return string(refType) + "/" + util.PathEscapeSegments(ref.ShortName())
 }
+
+func ParseRefSuffix(ref string) (string, string) {
+	// Partially support https://git-scm.com/docs/gitrevisions
+	if idx := strings.Index(ref, "@{"); idx != -1 {
+		return ref[:idx], ref[idx:]
+	}
+	if idx := strings.Index(ref, "^"); idx != -1 {
+		return ref[:idx], ref[idx:]
+	}
+	return ref, ""
+}