Older Gits talked about "updating" a file to add its content to the
index, but this terminology is confusing for new users. "to stage" is far
more intuitive and already used in e.g. the "git stage" command name.
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr> Signed-off-by: Junio C Hamano <gitster@pobox.com>