show-branch: take default arguments from configuration file.

show-branch: take default arguments from configuration file.

This lets showbranch.default multivalued configuration item to
be used as the default set of parameters to git-show-branch when
none is given on the command line.

I keep many topic branches (e.g. zzz/pack, net/misc) and
branches used only as a reference under subdirectories
(e.g. hold/{html,man,todo} track the same from git.git, but
clutters the show-branch output when shown along with the main
development; ko/master tracks what I have pushed out already and
refetched from the kernel.org server), and often run:

	$ git show-branch ko/master heads/*

to view only the ko/master head and branches I keep immediately
under $GIT_DIR/refs/heads.  With this change, I can have this in
my $GIT_DIR/config file:

	[showbranch]
		default = ko/master
		default = heads/*

and say

	$ git show-branch

Signed-off-by: Junio C Hamano <junkio@cox.net>