Do not talk about "diff" in rev-list documentation.

Since 8c02eee (git-rev-list(1): group options; reformat; document more
options, 2006-09-01), git-rev-list documentation talks as if it supports
any kind of diff output.  It doesn't.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index c953d1c..abf3420 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -90,6 +90,7 @@
 This implies the '--topo-order' option by default, but the
 '--date-order' option may also be specified.
 
+ifndef::git-rev-list[]
 Diff Formatting
 ~~~~~~~~~~~~~~~
 
@@ -119,6 +120,7 @@
 -t::
 
 	Show the tree objects in the diff output. This implies '-r'.
+endif::git-rev-list[]
 
 Commit Limiting
 ~~~~~~~~~~~~~~~