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: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
diff --git a/git-gui.sh b/git-gui.sh
index 85fbc02..549f59b 100755
--- a/git-gui.sh
+++ b/git-gui.sh
@@ -388,6 +388,9 @@
 
 	if {![info exists _nice]} {
 		set _nice [_which nice]
+		if {[catch {exec $_nice git version}]} {
+			set _nice {}
+		}
 	}
 	if {$_nice ne {}} {
 		lappend cmd $_nice