push: change needlessly ambiguous example in error

push: change needlessly ambiguous example in error

Change an example push added in b55e677522 ("push: introduce new
push.default mode "simple"", 2012-04-24) to always mean the same thing
whether the current setting happens to be "simple" or not.

This error is only emitted under "simple", but message is explaining
to the user that they can get two sorts of different behaviors by
these two invocations.

Let's use "git push <remote> HEAD" which always means push the current
branch name to that remote, instead of "git push <remote>
<current-branch-name>" which will do that under "simple", but is not
guaranteed to do under "upstream".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>