branch: warn and refuse to set a branch as a tracking branch of itself.

branch: warn and refuse to set a branch as a tracking branch of itself.

Previous patch allows commands like "git branch --set-upstream foo foo",
which doesn't make much sense. Warn the user and don't change the
configuration in this case. Don't die to let the caller finish its job in
such case.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>