emacs: use the originating buffer's working directory for pipe

Currently notmuch-show-pipe-message runs the command in the working
directory of the *notmuch-pipe* buffer if it exists, and the current
buffer's working directory (which is inherited to the new
*notmuch-pipe* buffer) otherwise. This is all very surprising to the
user, and it's difficult to know or change where the command will be
run.

Always use the current show buffer's working directory for piping. The
user can check that with M-x pwd and change it with M-x cd. This is
consistent with notmuch-show-pipe-part.