Properly handle pushing a branch without a remote.

Before, this did "git push remote branch-name:nil".
Now it does "git push remote branch-name".
Git automatically sets up the remote.

magit.el

@@ -3042,7 +3042,10 @@ typing and automatically refreshes the status buffer."
     (if (and (not branch-remote)
 	     (not current-prefix-arg))
 	(magit-set push-remote "branch" branch "remote"))
-    (magit-run-git-async "push" "-v" push-remote (format "%s:%s" branch ref-branch))))
+    (magit-run-git-async "push" "-v" push-remote
+                         (if ref-branch
+                             (format "%s:%s" branch ref-branch)
+                           branch))))
 
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