Documentation: enable compat-mode for Asciidoctor

Asciidoctor 1.5.0 and later have a compatibility mode that makes it more
compatible with some Asciidoc syntax, notably the single and double
quote handling.  While this doesn't affect any of our current
documentation, it would be beneficial to enable this mode to reduce the
differences between AsciiDoc and Asciidoctor if we make use of those
features in the future.

Since this mode is specified as an attribute, if a version of
Asciidoctor doesn't understand it, it will simply be ignored.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -181,6 +181,7 @@ ASCIIDOC = asciidoctor
 ASCIIDOC_CONF =
 ASCIIDOC_HTML = xhtml5
 ASCIIDOC_DOCBOOK = docbook45
+ASCIIDOC_EXTRA += -acompat-mode
 ASCIIDOC_EXTRA += -I. -rasciidoctor-extensions
 ASCIIDOC_EXTRA += -alitdd='&\#x2d;&\#x2d;'
 DBLATEX_COMMON =