py3: stringify IOError/OSError without loosing local character

Follows up fa4d333cac58. An environment error may contain non-ascii characters
on Windows, which should be encoded to a platform-native string.

mercurial/branchmap.py

@@ -377,7 +377,7 b' class revbranchcache(object):'
                 self._rbcrevs[:] = data
             except (IOError, OSError) as inst:
                 repo.ui.debug("couldn't read revision branch cache: %s\n" %
-                              pycompat.bytestr(inst))
+                              util.forcebytestr(inst))
         # remember number of good records on disk
         self._rbcrevslen = min(len(self._rbcrevs) // _rbcrecsize,
                                len(repo.changelog))