Documentation: render revisions correctly under Asciidoctor

When creating a literal block from an indented block without any sort of
delimiters, Asciidoctor strips off all leading whitespace, resulting in
a misrendered chart.  Use an explicit literal block to indicate to
Asciidoctor that we want to keep the leading whitespace.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/revisions.txt

@@ -345,6 +345,7 @@ Here are a handful of examples using the Loeliger illustration above,
 with each step in the notation's expansion and selection carefully
 spelt out:
 
+....
    Args   Expanded arguments    Selected commits
    D                            G H D
    D F                          G H I J D F
@@ -367,3 +368,4 @@ spelt out:
 	  = B ^B^1 ^B^2 ^B^3
 	  = B ^D ^E ^F          B
    F^! D  = F ^I ^J D           G H D F
+....