Add config variable for specifying default --dirstat behavior

Add config variable for specifying default --dirstat behavior

The new diff.dirstat config variable takes the same arguments as
'--dirstat=<args>', and specifies the default arguments for --dirstat.
The config is obviously overridden by --dirstat arguments passed on the
command line.

When not specified, the --dirstat defaults are 'changes,noncumulative,3'.

The patch also adds several tests verifying the interaction between the
diff.dirstat config variable, and the --dirstat command line option.

Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>