format-patch: add commitListFormat config

format-patch: add commitListFormat config

Using "--cover-letter" we can tell format-patch to generate a cover
letter, in this cover letter there's a list of commits included in the
patch series and the format is specified by the "--cover-letter-format"
option. Would be useful if this format could be configured from the
config file instead of always needing to pass it from the command line.

Teach format-patch how to read the format spec for the cover letter from
the config files. The variable it should look for is called
format.commitListFormat.

Possible values:
  - commitListFormat is set but no string is passed: it will default to
    "[%(count)/%(total)] %s"

  - if a string is passed: will use it as a format spec. Note that this
    is either "shortlog" or a format spec prefixed by "log:"
    e.g."log:%s (%an)"

  - if commitListFormat is not set: it will default to the shortlog
    format.

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