transplant: remove a mutable default argument

transplant: remove a mutable default argument

Mutable default arguments are know to the state of California to cause bugs.

--- a/hgext/transplant.py	Thu Sep 24 00:50:06 2015 -0700
+++ b/hgext/transplant.py	Thu Sep 24 00:52:21 2015 -0700
@@ -117,8 +117,10 @@
                 return True
         return False
 
-    def apply(self, repo, source, revmap, merges, opts={}):
+    def apply(self, repo, source, revmap, merges, opts=None):
         '''apply the revisions in revmap one by one in revision order'''
+        if opts is None:
+            opts = {}
         revs = sorted(revmap)
         p1, p2 = repo.dirstate.parents()
         pulls = []