doc: git-clone fix discrepancy between asciidoc and asciidoctor

Asciidoc.py does not have the concept of generalized roles, whereas asciidoctor interprets [foo]`blah` as blah with role foo in the synopsis, making in effect foo disappear in the output. Note that square brackets not directly followed by an inline markup do not define a role, which is why we do not have the issue on other parts of the documentation. In order to get a consistant result across asciidoctor and asciidoc.py, the hack is to use the {empty} entity to split the bracket part from the inline format part. Signed-off-by: Jean-Noël Avila <jn.avila@free.fr> Signed-off-by: Junio C Hamano <gitster@pobox.com>