patchgit-gui: check whether systems nice command works or disable it

> This fixes issue 394 from msysgit. It seems that the Gnuwin32 project
> provides a nice command but it returns a "not implemented" error. To
> help users we now try to execute once and disable it in case it fails.
>
> Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
> ---
>
> see: http://code.google.com/p/msysgit/issues/detail?id=394
> for more information.
>
> cheers Heiko
>
>  git-gui/git-gui.sh |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
>
> diff --git a/git-gui/git-gui.sh b/git-gui/git-gui.sh
> index 1fb3cbf..eec2dc9 100755
> --- a/git-gui/git-gui.sh
> +++ b/git-gui/git-gui.sh
> @@ -388,6 +388,9 @@ proc _lappend_nice {cmd_var} {
>  
>  	if {![info exists _nice]} {
>  		set _nice [_which nice]
> +		if {[catch {exec $_nice git version}]} {
> +			set _nice {}
> +		}

>  	}
>  	if {$_nice ne {}} {
>  		lappend cmd $_nice
> -- 
> 1.7.0.rc1.7.gc0da5

> > diff --git a/git-gui/git-gui.sh b/git-gui/git-gui.sh
> > index 1fb3cbf..eec2dc9 100755
> > --- a/git-gui/git-gui.sh
> > +++ b/git-gui/git-gui.sh
> > @@ -388,6 +388,9 @@ proc _lappend_nice {cmd_var} {
> >  
> >  	if {![info exists _nice]} {
> >  		set _nice [_which nice]
> > +		if {[catch {exec $_nice git version}]} {
> > +			set _nice {}
> > +		}
> 
> Where does the output from this "git version" invocation go?