shortlog: support --pretty=format: option

shortlog: support --pretty=format: option

With this patch, the user can override the default setting, to print
the commit messages using a user format instead of the onelines of the
commits.  Example:

	$ git shortlog --pretty='format:%s (%h)' <commit>..

Note that shortlog will only respect a user format setting, as the other
formats do not make much sense.

Wished for by Andrew Morton.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>