Documentation: use {asterisk} in rev-list-options.txt when needed

Documentation: use {asterisk} in rev-list-options.txt when needed

Text between two '*' is emphasized in AsciiDoc and makes explanations in
rev-list-options.txt on glob-related options very confusing, as the
rendered text would be missing two asterisks and the text between them
would be emphasized instead.

Use '{asterisk}' where needed to make them show up as asterisks in the
rendered text.

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