bundle: allow bundlerepo to support alternative manifest implementations With our treemanifest logic, the manifests are no longer transported as part of the changegroup and are no longer stored in a revlog. This means the self.manifestlog line in bundlerepo.filestart no longer calls _constructmanifest, and therefore does not consume the manifest portion of the changegroup, which means filestart is not populated and we result in an infinite loop. The fix is to make filestart aware that self.manifestlog might not consume the changegroup part, and consume it manually if necessary. There's currently no way to test this in core, but our treemanifest extension has tests to cover this. Differential Revision: https://phab.mercurial-scm.org/D1329