phabricator: follow-up phab auth improvements with backwards compat mode We'll rip this out before we ship 4.6, but this gives people a window to migrate. Differential Revision: https://phab.mercurial-scm.org/D2703

#require clang-libfuzzer test-repo
  $ cd $TESTDIR/../contrib/fuzz
  $ make
Just run the fuzzer for five seconds to verify it works at all.
  $ ./bdiff -max_total_time 5