Defining USE_ASCIIDOCTOR=1 when building Git uses asciidoctor over
asciidoc when generating DocBook and man page documentation. However,
the contrib/subtree module does not presently honour that flag.
This causes a build failure when asciidoc is not present on the build
system. Instead, adapt the main Documentation/Makefile logic to use
asciidoctor when requested.
Signed-off-by: A. Wilcox <AWilcox@Wilcox-Tech.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>