Disable asciidoc 8.4.1+ semantics for `{plus}` and friends

Disable asciidoc 8.4.1+ semantics for `{plus}` and friends

asciidoc 8.4.1 changed the semantics of inline backtick quoting so
that they disable parsing of inline constructs, i.e.,

  Input:	`{plus}`
  Pre 8.4.1:	+
  Post 8.4.1:	{plus}

Fix this by defining the asciidoc attribute 'no-inline-literal'
(which, per the 8.4.1 changelog, is the toggle to return to the old
behaviour) when under ASCIIDOC8.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>