bundle2: convert ints to strings using pycompat.bytestring()
Fixes some Python 3 regressions.
We don't use %d here because the part id is actually an
Optional[int]. It should always be initialized to a non-None value by
the time this code executes, but we shouldn't blindly depend on that
being the case.
Differential Revision: https://phab.mercurial-scm.org/D272
(binary:application/octet-stream)