Documentation: don't link to example mail addresses

Documentation: don't link to example mail addresses

Email addresses in documentation are converted into mailto: hyperlinks
in the HTML output and footnotes in man pages.  This isn't desirable for
cases where the address is used as an example and is not valid.

Particularly annoying is the example "jane@laptop.(none)" which appears
in git-shortlog(1) as "jane@laptop[1].(none)", with note 1 saying:

	1. jane@laptop
	   mailto:jane@laptop

Fix this by escaping these email addresses with a leading backslash, to
prevent Asciidoc expanding them as inline macros.

In the case of mailmap.txt, render the address monospaced so that it
matches the block examples surrounding that paragraph.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>