Define linkgit macro in [macros] section

Starting with asciidoc 8.3.0 linkgit macro is no longer recognized by
asciidoc and user guide suggests
(http://www.methods.co.nz/asciidoc/userguide.html#_macro_definitions)
that macros are supposed to be defined in [macros] section. I'm not
sure whether undefined linkgit macro was working by pure chance or it
is a regression in asciidoc 8.3.0, but this patch adds proper
definition for the linkgit macro, allowing it to work on 8.3.0.

Signed-off-by: Alexey Borzenkov <snaury@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/asciidoc.conf

@@ -7,6 +7,9 @@
 # Show GIT link as: <command>(<section>); if section is defined, else just show
 # the command.
 
+[macros]
+(?su)[\\]?(?P<name>linkgit):(?P<target>\S*?)\[(?P<attrlist>.*?)\]=
+
 [attributes]
 asterisk=&#42;
 plus=&#43;