Documentation: use 8-space tabs with Asciidoctor

Asciidoctor expands tabs at the beginning of a line. However, it does not expand them into 8 spaces by default. Since we use 8-space tabs, tell Asciidoctor that we want 8 spaces by setting the tabsize attribute. This ensures that our ASCII art renders properly. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>