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