convert/gnuarch: robustify cat-log retrieval GNU Arch used to scale very poorly when revision number was increasing. This was mostly caused by the huge amount of cat-log it has to scan/read through to keep track of all patches that were merged in a given revision. In order to improve things, cat-log prunning was a common admin task that would accelerate cat-log parsing at the expense of unreachabe locally stored cat-logs. However, these missing cat-logs are still available in the archive. So try to get them from the archive as a fallback solution.