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/smartset.py

@@ -670,7 +670,7 b' class addset(abstractsmartset):'
         if self._ascending:
             attr = 'fastasc'
         else:
-            attr = b'fastdesc'
+            attr = 'fastdesc'
         it = getattr(self, attr)
         if it is not None:
             return it()