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
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3