Backticks are emphasized through monospaced styling in the HTML
version of Git documentation. But they were left unstyled in the
manual pages.

To make the man pages more comfortably read, `MAN_BOLD_LITERAL` was
added by 5121a6d (Documentation: option to render literal text as
bold for manpages, 2009-03-27).  It allowed the user to build the
manpages with literals in bold style.

For precaution it was not set by default back then.

Since 79c461d (docs: default to more modern toolset, 2010-11-19), it
is assumed ASCIIDOC 8 and at least docbook-xsl 1.73 are used, so the
need for compatibility concern is much lessor now.

Remove `MAN_BOLD_LITERAL`, and typeset literals as bold by default .
Add `NO_MAN_BOLD_LITERAL`, a new Makefile option, disabling this
feature when defined.

Signed-off-by: Erwan MATHONIERE <erwan.mathoniere@grenoble-inp.org>
Signed-off-by: Samuel GROOT <samuel.groot@grenoble-inp.org>
Signed-off-by: Tom RUSSELLO <tom.russello@grenoble-inp.org>
Signed-off-by: Matthieu MOY <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>