setup: clarify error messages for file/revisions ambiguity

setup: clarify error messages for file/revisions ambiguity

The previous "Use '--' to separate filenames from revisions" may sound
obvious for an old-time Unix user, but does not make it clear how to use
this '--'. In addition to mentionning this '--', give an idea of what the
new command should look like.

Ideally, we could provide cut-and-paste ready commands based on the
command that just failed, but we have no easy access to argv[] in this
place of the code.

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