contrib: pick: move M-RET to prefix-arg RET

contrib: pick: move M-RET to prefix-arg RET

In pick the user has the option of showing the selected message in a
subpane (the message pane) or in the full frame. This is customisable
using the variable notmuch-pick-show-out. At the moment RET is bound
to the default option and M-RET the other option. This is
messy and involves tricks to make sure the keymap is setup at the
right time.

This changes this to prefix-arg RET for the other option which
simplifies the code and makes things cleaner.