error: implement __str__ on RevlogError to fix some output defects on Py3

We open-code encoding.unimethod here to avoid cycles, and do a local
import of encoding when someone str()s a RevlogError. It's not my
favorite solution, but it gets the job done.

Differential Revision: https://phab.mercurial-scm.org/D5426

mercurial/error.py

@@ -44,6 +44,14 class StorageError(Hint, Exception):
 class RevlogError(StorageError):
     __bytes__ = _tobytes
 
+    def __str__(self):
+        # avoid cycle, and directly implement unimethod for this
+        # __str__ to allow delaying the import of encoding until
+        # someone actually wants the __str__ of a RevlogError (which
+        # should be very rare).
+        from . import encoding
+        return encoding.unifromlocal(_tobytes(self))
+
 class FilteredIndexError(IndexError):
     __bytes__ = _tobytes