Allow pickaxe and diff-filter options to be used by git log.

Allow pickaxe and diff-filter options to be used by git log.

Handle the -S option when passed to git log such that only the
appropriate commits are displayed.  Also per Junio's comments, do
the same for "--diff-filter", so that it too can be used as an option
to git log.  By default no patch or diff information is displayed.

Signed-off-by: Sean Estabrooks <seanlkml@sympatico.ca>
Signed-off-by: Junio C Hamano <junkio@cox.net>