Documentation: refactor common operations into variables

The Makefile performs several very similar tasks to convert AsciiDoc
files into either HTML or DocBook.  Move these items into variables to
reduce the duplication.

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

Asciidoctor provides an extension implementing a backend-independent
macro for dealing with manpage links just like the linkgit macro.  As
this is more likely to be up-to-date with future changes in Asciidoctor,
prefer using it over reimplementing in Git.

This reverts commit 773ee47c2b9c691d9758b2bea6cac10e3f0c4e12.

Signed-off-by: Junio C Hamano <gitster@pobox.com>