git-gui: Fix switch statement in lib/merge.tcl

0aea284 (Make Ctrl-T safe to use for conflicting files) introduced a new
case, but forgot the '-' to indicate that it shares the body with the
subsequent case label.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

lib/merge.tcl

@@ -40,7 +40,7 @@ The rescan will be automatically started now.
 		_O {
 			continue; # and pray it works!
 		}
-		_U
+		_U -
 		U? {
 			error_popup [mc "You are in the middle of a conflicted merge.