shortlog: extract `shortlog_finish_setup()`

shortlog: extract `shortlog_finish_setup()`

Extract a function which finishes setting up the shortlog struct for
use. The caller in `make_cover_letter()` does not care about trailer
sorting, so it isn't strictly necessary to add a call there in this
patch.

But the next patch will add additional functionality to the new
`shortlog_finish_setup()` function, which the caller in
`make_cover_letter()` will care about.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>