revlog: correct type in check to verify rawtext is immutable

This fixes far more failures than I feel like it has any right to, so
there's clearly some subtle interaction between self._cache and other
parts of this code. :(

Differential Revision: https://phab.mercurial-scm.org/D1906

mercurial/revlog.py

@@ -2099,7 +2099,7 b' class revlog(object):'
         if alwayscache and rawtext is None:
             rawtext = deltacomputer._buildtext(revinfo, fh)
 
-        if type(rawtext) == str: # only accept immutable objects
+        if type(rawtext) == bytes: # only accept immutable objects
             self._cache = (node, curr, rawtext)
         self._chainbasecache[curr] = chainbase
         return node