shortlog: handle multi-line subjects like log --pretty=oneline et. al. do

shortlog: handle multi-line subjects like log --pretty=oneline et. al. do

The commit message parser of git shortlog used to treat only the first
non-empty line of the commit message as the subject.  Other log commands
(e.g. --pretty=oneline) show the whole first paragraph instead (unwrapped
into a single line).

For consistency, this patch borrows format_subject() from pretty.c to
make shortlog do the same.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>