largefiles: remove a mutable default argument

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

hgext/largefiles/overrides.py

@@ -1172,8 +1172,10 b' def overridesummary(orig, ui, repo, *pat'
     finally:
         repo.lfstatus = False
 
-def scmutiladdremove(orig, repo, matcher, prefix, opts={}, dry_run=None,
+def scmutiladdremove(orig, repo, matcher, prefix, opts=None, dry_run=None,
                      similarity=None):
+    if opts is None:
+        opts = {}
     if not lfutil.islfilesrepo(repo):
         return orig(repo, matcher, prefix, opts, dry_run, similarity)
     # Get the list of missing largefiles so we can remove them