add format.usefncache config option (default is true)

This enables for example "hg clone --config format.usefncache=0" which
creates a non-fncache repository, which then can be read by Mercurial
versions before 7946503ec76e.

mercurial/localrepo.py

@@ -35,7 +35,8 b' class localrepository(repo.repository):'
                 if parentui.configbool('format', 'usestore', True):
                     os.mkdir(os.path.join(self.path, "store"))
                     requirements.append("store")
-                    requirements.append("fncache")
+                    if parentui.configbool('format', 'usefncache', True):
+                        requirements.append("fncache")
                     # create an invalid changelog
                     self.opener("00changelog.i", "a").write(
                         '\0\0\0\2' # represents revlogv2