bundle2.unbundlepart: implement seek() This implements a seek() method for unbundlepart. This allows on-disk bundle2 parts to behave enough like files for bundlerepo to handle them. A future patch will add support for bundlerepo to read the bundle2 files that are written when the experimental.strip-bundle2-version config option is used.

bundle2.unbundlepart: tell() implementation Keep track of how many bytes we've returned from read(); fairly straightforward.

bundle2.unbundlepart: keep an index of chunks and their locations In order to make unbundlepart seekable, we need to keep a record of where the chunks are so that we can go back to the correct point.