Documentation: bold literals in man

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>

Documentation/Makefile

@@ -146,7 +146,7 @@ else
 	ASCIIDOC_EXTRA += -a git-asciidoc-no-roff
 	endif
 endif
-ifdef MAN_BOLD_LITERAL
+ifndef NO_MAN_BOLD_LITERAL
 XMLTO_EXTRA += -m manpage-bold-literal.xsl
 endif
 ifdef DOCBOOK_SUPPRESS_SP