bundle2: only seek to beginning of part in bundlerepo For reasons still not yet fully understood by me, bundlerepo requires its changegroup bundle2 part to be seeked to beginning after part iteration. As far as I can tell, it is the only bundle2 part consumer that relies on this behavior. This seeking was performed in the generic iterparts() API. Again, I don't fully understand why it was here and not in bundlerepo. Probably historical reasons. What I do know is that all other bundle2 part consumers don't need this special behavior (assuming the tests are comprehensive). So, we move the code from bundle2's iterparts() to bundlerepo's consumption of iterparts(). Differential Revision: https://phab.mercurial-scm.org/D1389