git-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: NHeiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

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