add -u: get rid of "treewideupdate" configuration

add -u: get rid of "treewideupdate" configuration

Thanks to the magic ":/" pathspec, it is much easier to invoke both
tree-wide operation and limited-to-cwd operation on demand from the
command line.  Only the downside of the configuration variable, namely,
it makes git behave differently depending on who you are and in which
repository you are using it, hence making it harder to help and/or
teach others, remains.

Remove the configuration variable, and adjust the warning message.

Signed-off-by: Junio C Hamano <gitster@pobox.com>