convert/gnuarch: follow continuation-of revisions Built on top of previous patches: - continuation-of parsing - registered archives retrieval - use of fully qualified revisions This allows the converter scanning for more source revisions following the tree versions 'leaked' through the continuation-of informations. Coupled with the registered archives retrieval, this makes possible to decide to follow such a hint or stop scanning for more revisions. This also implies some changes in the retrieval of some base-0 revisions when they're continuation-of other revisions, in that case a 'replay' will work where a simple 'get' fails because the dir exists already. I found the code dealing with 'replay' quite good as it has already a fallback to 'get' in the error path.