safehasattr: pass attribute name as string instead of bytes

This is a step toward replacing `util.safehasattr` usage with plain `hasattr`.
The builtin function behave poorly in Python2 but this was fixed in Python3.

These change are done one by one as they tend to have a small odd to trigger
puzzling breackage.

mercurial/manifest.py

@@ -1619,7 +1619,7 b' class manifestrevlog:'
 
     def _setupmanifestcachehooks(self, repo):
         """Persist the manifestfulltextcache on lock release"""
-        if not util.safehasattr(repo, b'_wlockref'):
+        if not util.safehasattr(repo, '_wlockref'):
             return
 
         self._fulltextcache._opener = repo.wcachevfs