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>

Documentation/Makefile

@@ -184,7 +184,7 @@ ASCIIDOC = asciidoctor
 ASCIIDOC_CONF =
 ASCIIDOC_HTML = xhtml5
 ASCIIDOC_DOCBOOK = docbook45
-ASCIIDOC_EXTRA += -acompat-mode
+ASCIIDOC_EXTRA += -acompat-mode -atabsize=8
 ASCIIDOC_EXTRA += -I. -rasciidoctor-extensions
 ASCIIDOC_EXTRA += -alitdd='&\#x2d;&\#x2d;'
 DBLATEX_COMMON =