format-patch: --commit-list-format without prefix

format-patch: --commit-list-format without prefix

Having to prefix a custom format-string with "log:" when passed from the
CLI can be annoying. It would be great if this prefix wasn't required.

Teach make_cover_letter() to accept custom format-strings without the
"log:" prefix if a placeholder is detected.

Note that both here and in "git log --format" the check is done naively
by just checking for the presence of a '%'.

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