push: better error messages when push.default = tracking

push: better error messages when push.default = tracking

A common scenario is to create a new branch and push it (checkout -b &&
push [--set-upstream]). In this case, the user was getting "The current
branch %s has no upstream branch.", which doesn't help much.

Provide the user a command to push the current branch. To avoid the
situation in the future, suggest --set-upstream.

While we're there, also improve the error message in the "detached HEAD"
case. We mention explicitly "detached HEAD" since this is the keyword to
look for in documentations.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>