emacs: Add a space after completed tag operations

emacs: Add a space after completed tag operations

Previously, when a user fully completed a tag operation, they had to
press space to begin entering another tag operation.  This is
different from, say, shell file name completion, which typically
inserts a space after an unambiguous completion under the assumption
that the user will want to enter more input.

This patch tweaks `notmuch-read-tag-changes' to act more like shell
file name completion: after an unambiguous tag completion, it now
inserts a space, ready and waiting for another tagging operation from
the user.  This is backwards-compatible with old habits, since there's
no harm in putting an extra space.