upgrade: sniff for filelog type

The upgrade code should never encounter a vanilla revlog
instance: only changelog, manifestrevlog, and filelog should
be seen.

The previous code assumed !changelog & !manifestrevlog meant
file data. So this change feels pretty safe. If nothing else, it
will help tease out typing issues.

Differential Revision: https://phab.mercurial-scm.org/D3152

mercurial/upgrade.py

@@ -480,11 +480,13 def _copyrevlogs(ui, srcrepo, dstrepo, t
             mrevcount += len(rl)
             msrcsize += datasize
             mrawsize += rawsize
-        elif isinstance(rl, revlog.revlog):
+        elif isinstance(rl, filelog.filelog):
             fcount += 1
             frevcount += len(rl)
             fsrcsize += datasize
             frawsize += rawsize
+        else:
+            error.ProgrammingError('unknown revlog type')
 
     if not revcount:
         return