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>