format-patch: rename --cover-letter-format option

format-patch: rename --cover-letter-format option

To align the name of the configuration variable and the name of the
command line option, either one should change name. By changing the name
of the option we get the added benefit of having --cover-<TAB> expand to
--cover-letter without ambiguity.

If the user gives the --cover-letter-format option it would be
reasonable to expect that the user wants to generate the cover letter
despite not giving --cover-letter.

Rename --cover-letter-format to --commit-list-format and make it imply
--cover-letter unless --no-cover-letter is given.

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