notmuch.el: quote args in notmuch-show to facilitate remote use

Put single-quotes around the argument of the `show --entire-thread' command
in notmuch-show. This change should have no effect on normal usage.
However, it allows us to use the notmuch.el client with a remote notmuch
binary and database over ssh (by, e.g., setting `notmuch-command' to a
simple shell script). Without the quotes, ssh will not send the command
properly.

One very simple example script is as follows. (Note that it requires
keypair login to the ssh server.)

        #!/bin/sh

        SSH_BIN="/path/to/local/ssh"
        NOTMUCH_HOST="my.remote.server"
        NOTMUCH_REMOTE_PATH="/path/to/remote/notmuch"

        $SSH_BIN $NOTMUCH_HOST $NOTMUCH_REMOTE_PATH $@