Emacs:git-messenger: change keybinding C-c v p

Change key binding 'git-messenger:popup-message' from C-x v p ->
C-c v p. This is in line with key bindings for git-gutter+.

Tested-by: Ratheesh S <ratheeshreddy@gmail.com>
Signed-off-by: Ratheesh S <ratheeshreddy@gmail.com>