Previously, `hg log -r 'bundle()' -R bundle.hg` was failing for me when run from
source, complaining about an unknown parent, when the system installed `hg`
didn't.  Some debugging showed the index was 0 length.  It turned out that I
didn't have the C extensions compiled, which a simple `hg log -r .` was able to
indicate.  The problem being that the RequirementError got handled by RepoError,
which uses an empty directory as a fallback to process the bundle.