emacs: notmuch-address-expand-name: use the actual initial-input

emacs: notmuch-address-expand-name: use the actual initial-input

Users may type some text into the buffer on an address line, before
actually invoking address completion.  We now use that text as the
initial input when we begin address completion.

Previously we did knowingly replace the actual initial input with some
completion candidate that happens to match. Which candidate is used is
essentially random, at least when the actual initial input is short.
As a result users very often had to begin completion by deleting the
less than helpful "initial input".