Documentation: remove {show,whatchanged}.difftree config options

This removes, from the documentation and the bash completion script, the
two config options that were introduced by the git-whatchanged.sh script
and lost in the C rewrite. Today, we can use aliases as an alternative.

Signed-off-by: Olivier Marin <dkr@freesurf.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1041,7 +1041,6 @@ _git_config ()
 		pull.octopus
 		pull.twohead
 		repack.useDeltaBaseOffset
-		show.difftree
 		showbranch.default
 		tar.umask
 		transfer.unpackLimit
@@ -1050,7 +1049,6 @@ _git_config ()
 		user.name
 		user.email
 		user.signingkey
-		whatchanged.difftree
 		branch. remote.
 	"
 }