bug fixes to emacs notmuch-tag-completion function

[Jameson Graef Rollins]

I had originally included this fixes as part of a more involved series
to fix up tagging functions [0].  However, these are actually bug
fixes that stand alone and should not be bogged down in a series
that's likely going to take a while to come to fruition.  The changes
here are pretty trivial, so hopefully they can get pushed quickly and
I'll rework the rest of tagging fix up series on top of them.

jamie.

[0] id:"1333354853-25729-1-git-send-email-jrollins at finestructure.net"