revlog: fix error about unknown compression format in py3

In py2, the error is something like:
abort: unknown compression type 'x'!

In py3, we get the following unhelpful message:
abort: unknown compression type <memory at 0x7f4650b5cdc8>!

Switch to something like:
abort: unknown compression type 78!

Differential Revision: https://phab.mercurial-scm.org/D10318

mercurial/revlog.py

@@ -13,6 +13,7 b' and O(changes) merge between branches.'
 
 from __future__ import absolute_import
 
+import binascii
 import collections
 import contextlib
 import errno
@@ -2296,7 +2297,9 b' class revlog(object):'
                 compressor = engine.revlogcompressor(self._compengineopts)
                 self._decompressors[t] = compressor
             except KeyError:
-                raise error.RevlogError(_(b'unknown compression type %r') % t)
+                raise error.RevlogError(
+                    _(b'unknown compression type %s') % binascii.hexlify(t)
+                )
 
         return compressor.decompress(data)