git: configure asciidoctor usage via `config.mak`
git: configure asciidoctor usage via `config.mak`

By configuring it in `config.mak`, interactive `make` calls in the
`src/git/` subdirectory will also use asciidoctor (fast!) and not fall
back to asciidoc (slow!).

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

git-for-windows/MINGW-packages,dscho/MINGW-packages,vcgato29/MINGW-packages,git-for-windows/MINGW-packages,dscho/MINGW-packages,vcgato29/MINGW-packages,dscho/MINGW-packages,JanSchulz/MINGW-packages,dscho/MINGW-packages,git-for-windows/MINGW-packages,vcgato29/MINGW-packages,JanSchulz/MINGW-packages,dscho/MINGW-packages,JanSchulz/MINGW-packages