magit-dispatch-popup: Use magit-show-refs-popup
magit-dispatch-popup: Use magit-process

magit-dispatch-popup wasn't updated when magit-display-process was
renamed magit-process-display-buffer [1]. Later, the command
magit-process was added and magit-process-display-buffer was turned into
a non-interactive function, so use magit-process for the dispatch popup.

[1] bca9560d763b683a4ef98377448480fd93aa0f87
[2] cff64191fdf3f010b77469673ad299f5019bbf96
Merge pull request #1652 from kyleam/n/update-dispatch

Update magit-dispatch-popup