config option log.showroot to show the diff of root commits

This allows one to see a root commit as a diff in commands like git-log,
git-show and git-whatchanged.

Signed-off-by: Peter Baumann <Peter.B.Baumannn@stud.informatik.uni-erlangen.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t4013-diff-various.sh

@@ -73,6 +73,7 @@ test_expect_success setup '
 	for i in 1 2; do echo $i; done >>dir/sub &&
 	git update-index file0 dir/sub &&
 
+	git repo-config log.showroot false &&
 	git commit --amend &&
 	git show-branch
 '