emacs: avoid passing around some redundant information

emacs: avoid passing around some redundant information

When running "notmuch" we use its full path but when displaying the
command to the user we show just its name for readability reasons.
Avoid passing around both representations because it is very easy
to get the name from the path.

Notmuch itself uses the involved functions just for "notmuch" but
there might be extensions that use them for other executable so we
forgo other potential simplifications.