merge: prevent simplemerge from mutating label list

simplemerge was using list.pop() to remove items from the labels list. This
mutated the list and made it unusable by other calls (for instance, it might be
used in both the premerge and actual merge stages).

mercurial/simplemerge.py

@@ -416,11 +416,11 b' def simplemerge(ui, local, base, other, '
     name_a = local
     name_b = other
     labels = opts.get('label', [])
-    if labels:
-        name_a = labels.pop(0)
-    if labels:
-        name_b = labels.pop(0)
-    if labels:
+    if len(labels) > 0:
+        name_a = labels[0]
+    if len(labels) > 1:
+        name_b = labels[1]
+    if len(labels) > 2:
         raise util.Abort(_("can only specify two labels."))
 
     try: