Define linkgit macro in [macros] section

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>