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>

routers/api/v1/repo/pull.go

@@ -1062,19 +1062,11 @@ func MergePullRequest(ctx *context.APIContext) {
 // parseCompareInfo returns non-nil if it succeeds, it always writes to the context and returns nil if it fails
 func parseCompareInfo(ctx *context.APIContext, compareParam string) (result *git_service.CompareInfo, closer func()) {
 	baseRepo := ctx.Repo.Repository
-	compareReq, err := common.ParseCompareRouterParam(compareParam)
-	switch {
-	case errors.Is(err, util.ErrInvalidArgument):
-		ctx.APIError(http.StatusBadRequest, err.Error())
-		return nil, nil
-	case err != nil:
-		ctx.APIErrorInternal(err)
-		return nil, nil
-	}
+	compareReq := common.ParseCompareRouterParam(compareParam)
 
 	// remove the check when we support compare with carets
-	if compareReq.CaretTimes > 0 {
-		ctx.APIError(http.StatusBadRequest, "Unsupported compare syntax with carets")
+	if compareReq.BaseOriRefSuffix != "" {
+		ctx.APIError(http.StatusBadRequest, "Unsupported comparison syntax: ref with suffix")
 		return nil, nil
 	}