format-patch: introduce format.outputDirectory configuration

format-patch: introduce format.outputDirectory configuration

We can pass -o/--output-directory to the format-patch command to store
patches in some place other than the working directory. This patch
introduces format.outputDirectory configuration option for same
purpose.

The case of usage of this configuration option can be convenience
to not pass every time -o/--output-directory if an user has pattern
to store all patches in the /patches directory for example.

The format.outputDirectory has lower priority than command line
option, so if user will set format.outputDirectory and pass the
command line option, a result will be stored in a directory that
passed to command line option.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Stephen P. Smith <ischis2@cox.net>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>