pretty: factor out format_subject()

The next patch will use it.

In the version that was factored out, we can't rely on the len of the
struct strbuf to find out if a line separator needs to be added, as
it might already contain something.  Add a guard variable ("first")
instead.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>