Makefile: add check-docs exception for gitrevisions

The manpage was added in 1ed6f2c (Documentation: gitrevisions,
2010-07-05), but since it does not have a corresponding git command,
it needs an exception for check-docs.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2254,6 +2254,7 @@ check-docs::
 		documented,gitglossary | \
 		documented,githooks | \
 		documented,gitrepository-layout | \
+		documented,gitrevisions | \
 		documented,gittutorial | \
 		documented,gittutorial-2 | \
 		documented,git-bisect-lk2009 | \