format-patch: removing unconditional wrapping

format-patch: removing unconditional wrapping

Using format-patch with --commit-list-format different than shortlog,
causes the commit entry lines to wrap if they get longer than
MAIL_DEFAULT_WRAP (72 characters).

While this might be sensible for many when sending changes through
email, it forces this decision of wrapping on the user, reducing the
control granularity of --commit-list-format.

Teach generate_commit_list_cover() to respect commit entry line lengths
and place this wrapping rule on the "modern" preset format instead.

Signed-off-by: Mirko Faina <mroik@delayed.space>
Signed-off-by: Junio C Hamano <gitster@pobox.com>