Documentation: implement linkgit macro for Asciidoctor

Documentation: implement linkgit macro for Asciidoctor

AsciiDoc uses a configuration file to implement macros like linkgit,
while Asciidoctor uses Ruby extensions.  Implement a Ruby extension that
implements the linkgit macro for Asciidoctor in the same way that
asciidoc.conf does for AsciiDoc.  Adjust the Makefile to use it by
default.

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