git-gui: Show/hide "Sign Off" based on nocommitmsg option

If citool --nocommit is invoked we hide the Sign Off features, as
the commit message area is not editable.  But we really want the
selection tied to the message area's editing ability.

Suggested-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -2413,7 +2413,7 @@ if {[is_enabled multicommit] || [is_enabled singlecommit]} {
 
 	.mbar.commit add separator
 
-	if {![is_enabled nocommit]} {
+	if {![is_enabled nocommitmsg]} {
 		.mbar.commit add command -label [mc "Sign Off"] \
 			-command do_signoff \
 			-accelerator $M1T-S
@@ -2743,7 +2743,7 @@ pack .vpane.lower.commarea.buttons.incall -side top -fill x
 lappend disable_on_lock \
 	{.vpane.lower.commarea.buttons.incall conf -state}
 
-if {![is_enabled nocommit]} {
+if {![is_enabled nocommitmsg]} {
 	button .vpane.lower.commarea.buttons.signoff -text [mc "Sign Off"] \
 		-command do_signoff
 	pack .vpane.lower.commarea.buttons.signoff -side top -fill x