filelog: switch 'not len(filerevlog)' to 'not filerevlog'

A few places in the code use 'if not len(revlog)' to check if the revlog
exists. Replacing this with 'not filerevlog' allows alternative revlog
implementations to override __nonzero__ to handle this case without
implementing __len__.

mercurial/changegroup.py

@@ -359,7 +359,7 b' class bundle10(object):'
         msgfiles = _('files')
         for i, fname in enumerate(sorted(changedfiles)):
             filerevlog = repo.file(fname)
-            if not len(filerevlog):
+            if not filerevlog:
                 raise util.Abort(_("empty or missing revlog for %s") % fname)
 
             if fastpathlinkrev:

mercurial/cmdutil.py

@@ -1338,7 +1338,7 b' def _makegraphlogrevset(repo, pats, opts'
                 raise util.Abort(_('cannot follow file not in parent '
                                    'revision: "%s"') % f)
             filelog = repo.file(f)
-            if not len(filelog):
+            if not filelog:
                 # A zero count may be a directory or deleted file, so
                 # try to find matching entries on the slow path.
                 if follow: