git-gui: remove 'no such variable' for s error when encounter unknown file states

$s will be referenced in the error message. Which was broken since
"git-gui: Automatically update-index all included files before commit"
(bbe3b3b, 2006-11-16).

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
diff --git a/lib/commit.tcl b/lib/commit.tcl
index 7f459cd..83b3d9d 100644
--- a/lib/commit.tcl
+++ b/lib/commit.tcl
@@ -161,7 +161,8 @@
 	#
 	set files_ready 0
 	foreach path [array names file_states] {
-		switch -glob -- [lindex $file_states($path) 0] {
+		set s $file_states($path)
+		switch -glob -- [lindex $s 0] {
 		_? {continue}
 		A? -
 		D? -