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.

hgext/convert/gnuarch.py

@@ -73,7 +73,9 b' class gnuarch_source(converter_source, c'
             self.changes[rev] = self.gnuarch_rev(rev)
 
             # Read author, date and summary
-            catlog = self.run0('cat-log', '-d', self.path, rev)
+            catlog, status = self.run('cat-log', '-d', self.path, rev)
+            if status:
+                catlog = self.run0('cat-archive-log', rev)
             self._parsecatlog(catlog, rev)
 
             self.parents[rev] = child