bundlerepo: rename "bundlefilespos" variable and attribute
Strictly speaking, this variable tracks offsets within the
changegroup, not the bundle.
While we're here, mark a class attribute as private because
it is.
.. api::
Rename bundlerepo.bundlerepository.bundlefilespos to
_cgfilespos.
Differential Revision: https://phab.mercurial-scm.org/D1384