format-patch: add ability to use alt cover format

format-patch: add ability to use alt cover format

Often when sending patch series there's a need to clarify to the
reviewer what's the purpose of said series, since it might be difficult
to understand it from reading the commits messages one by one.

"git format-patch" provides the useful "--cover-letter" flag to declare
if we want it to generate a template for us to use. By default it will
generate a "git shortlog" of the changes, which developers find less
useful than they'd like, mainly because the shortlog groups commits by
author, and gives no obvious chronological order.

Give format-patch the ability to specify an alternative format spec
through the "--cover-letter-format" option. This option either takes
"shortlog", which is the current format, or a format spec prefixed with
"log:".

Example:
    git format-patch --cover-letter \
        --cover-letter-format="log:[%(count)/%(total)] %s (%an)" HEAD~3

    [1/3] this is a commit summary (Mirko Faina)
    [2/3] this is another commit summary (Mirko Faina)
    ...

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