emacs: Use single-quote not double-quote when constructing search

Commit 44982ab33295009137e3740e644e793a08629762 added some extra
quoting when constructing a search. A previous version of this patch
had used single-quotation marks (') while this version used
double-quotation marks (").

The intent of the extra quoting was to allow notmuch-command to be set
to a script invoking ssh.

What actually happens, however is that the extra quotation marks make
it all the way into the query string seen by Xapian. And the double
quotes trigger phrase searching, (which isn't desired here). The
side-effect of that is that the emacs code would fallback to an
unqalified query and display all threads with all messages open.

We fix that side-effect now by using single-quote characters, but
we'll want a better fix in the future to avoid Xapian seeing these
characters at all I think.