Make git-{pull,rebase} message without tracking information friendlier

Make git-{pull,rebase} message without tracking information friendlier

The current message is too long and at too low a level for anybody
to understand it if they don't know about the configuration format
already.

The text about setting up a remote is superfluous and doesn't help
understand or recover from the error that has happened.  Show the
usage more prominently and explain how to set up the tracking
information. If there is only one remote, that name is used instead
of the generic <remote>.

Also simplify the message we print on detached HEAD to remove
unnecessary information which is better left for the documentation.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>