git-gui: Make usage statement visible on Windows.

On Windows stdout and stderr are not connected to anything so the usage
statement is never shown to the user when an error is made with a command
line like 'git gui browser'. Use a messagebox on windows.
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -2845,7 +2845,13 @@ bind all <$M1B-Key-W> {destroy [winfo toplevel %W]}
 
 set subcommand_args {}
 proc usage {} {
-	puts stderr "usage: $::argv0 $::subcommand $::subcommand_args"
+	set s "usage: $::argv0 $::subcommand $::subcommand_args"
+	if {[tk windowingsystem] eq "win32"} {
+		wm withdraw .
+		tk_messageBox -icon info -title "Usage" -message $s
+	} else {
+		puts stderr $s
+	}
 	exit 1
 }