filectx: replace use of _filerev with _filenode

_filerev depends on the filelog implementation using revlogs and linkrevs.
Alternative implementations, like remotefilelog, do not have rev numbers, so
this call fails. Replacing it with _filenode means it doesn't rely on rev
numbers, and doesn't cost anything extra, since _filerev is using _filenode
under the hood anyway.

mercurial/context.py

@@ -789,7 +789,7 b' class basefilectx(object):'
         if fctx._customcmp:
             return fctx.cmp(self)
 
-        if (fctx._filerev is None
+        if (fctx._filenode is None
             and (self._repo._encodefilterpats
                  # if file data starts with '\1\n', empty metadata block is
                  # prepended, which adds 4 bytes to filelog.size().