largefiles: reduce redundant splitstandin/standin combination

assumptions:

    - 'lfutil.splitstandin(f)' strips 'f' of '.hglf/', if it can
    - 'lfile(f)' examines 'lfutil.standin(f) in manifest'
    - 'lfutil.standin(f)' adds '.hglf/' to 'f'

and

    - 'f' should start with '.hglf/' there, because it is already
      examined by 'lfutil.isstandin(f)'

then:

    lfile(lfutil.splitstandin(f))
      => lfile(<f without '.hglf/'>)
      => lfutil.standin(<f without '.hglf/'>) in manifest
      => <<f without '.hglf/'> with '.hglf/'> in manifest
      => f in manifest

hgext/largefiles/overrides.py

@@ -454,7 +454,7 def override_copy(orig, ui, repo, pats,
                 m._fmap = set(m._files)
                 orig_matchfn = m.matchfn
                 m.matchfn = lambda f: (lfutil.isstandin(f) and
-                                    lfile(lfutil.splitstandin(f)) and
+                                    (f in manifest) and
                                     orig_matchfn(lfutil.splitstandin(f)) or
                                     None)
                 return m