revlog: fix error message when data are missing

The error message the message was not using the requested offset, but the
adjusted offset to that read more data for improved caching. This resulted in
confusing error message.

Differential Revision: https://phab.mercurial-scm.org/D10591

mercurial/revlog.py

@@ -1556,7 +1556,7 b' class revlog(object):'
                     % (
                         self._indexfile if self._inline else self._datafile,
                         length,
-                        realoffset,
+                        offset,
                         len(d) - startoffset,
                     )
                 )