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.
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3