patchMakefile: 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>
> ---
>  Makefile |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 60a846e..74005dd 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -2259,6 +2259,7 @@ check-docs::
>  		documented,gitglossary | \
>  		documented,githooks | \
>  		documented,gitrepository-layout | \
> +		documented,gitrevisions | \
>  		documented,gittutorial | \
>  		documented,gittutorial-2 | \
>  		documented,git-bisect-lk2009 | \