Add merge arrow direction and update styling (#28523)

Close https://github.com/go-gitea/gitea/issues/28522

~Adds some [negative
margin](https://tailwindcss.com/docs/margin#using-negative-values)
helper css classes using tailwind's [prefix
syntax](https://tailwindcss.com/docs/configuration#prefix)~

### Before

![image](https://github.com/go-gitea/gitea/assets/12700993/5bdabf91-facd-41c8-8e36-e1535beb9409)

### After

![image](https://github.com/go-gitea/gitea/assets/12700993/a6f11f6f-9e64-45b6-b9d7-dfea53fbc6d7)

web_src/css/repo.css

@@ -1166,8 +1166,10 @@
   flex-wrap: wrap;
 }
 
-.repository .choose.branch .svg {
-  margin-right: 10px;
+.repository .choose .compare-separator {
+  width: 100%;
+  margin-top: -1rem;
+  text-align: center;
 }
 
 .repository.compare.pull .comment.form .content::before,