git-gui: Abort on not implemented branch switching.

I'm not currently ready to implement branch switching, so I'm just
going to punt on it for now.  :-)
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui

@@ -1634,7 +1634,7 @@ proc populate_branch_menu {m} {
 	foreach b $all_branches {
 		$m add radiobutton \
 			-label $b \
-			-command [list do_switch_branch $b] \
+			-command [list switch_branch $b] \
 			-variable current_branch \
 			-value $b \
 			-font font_ui
@@ -1643,6 +1643,10 @@ proc populate_branch_menu {m} {
 	}
 }
 
+proc switch_branch {b} {
+	error "NOT IMPLEMENTED"
+}
+
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 ## remote management