largefiles: remove a mutable default argument

largefiles: remove a mutable default argument

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

--- a/hgext/largefiles/overrides.py	Thu Sep 24 00:44:32 2015 -0700
+++ b/hgext/largefiles/overrides.py	Thu Sep 24 00:44:59 2015 -0700
@@ -289,13 +289,15 @@
         repo._repo.lfstatus = False
 
 def overridelog(orig, ui, repo, *pats, **opts):
-    def overridematchandpats(ctx, pats=(), opts={}, globbed=False,
+    def overridematchandpats(ctx, pats=(), opts=None, globbed=False,
             default='relpath', badfn=None):
         """Matcher that merges root directory with .hglf, suitable for log.
         It is still possible to match .hglf directly.
         For any listed files run log on the standin too.
         matchfn tries both the given filename and with .hglf stripped.
         """
+        if opts is None:
+            opts = {}
         matchandpats = oldmatchandpats(ctx, pats, opts, globbed, default,
                                        badfn=badfn)
         m, p = copy.copy(matchandpats)