Fix up "make doc"

Fix 'git-var.txt' and use "-b xhtml11" instead of "-b css-embedded" to
make asciidoc 7.0.1 happy.
diff --git a/Documentation/git-var.txt b/Documentation/git-var.txt
index 5a27d44..45b1006 100644
--- a/Documentation/git-var.txt
+++ b/Documentation/git-var.txt
@@ -1,5 +1,5 @@
 git-var(1)
-==================
+==========
 v0.1, July 2005
 
 NAME