Documentation/Makefile: add missing dependency on asciidoctor-extensions

Documentation/Makefile: add missing dependency on asciidoctor-extensions

asciidoctor-extensions.rb has never changed, but when it does -- such as
in the next commit --, it helps if the xml-files depend on it. We're
casting the net a bit too wide here, since we'll be rebuilding even with
AsciiDoc, which won't look at this file. But since this file changes so
rarely, that should be ok. It's better than missing a dependency.

Similarly, most of the html-files are produced directly from ".txt';
make the html-files too depend on asciidoctor-extensions.rb, both
the HTMLified manual pages as well as the user-manual that does use
.xml intermediary.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>