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

@@ -1127,7 +1127,7 b' class fullreposet(_spanset):'
         This boldly assumes the other contains valid revs only.
         """
         # other not a smartset, make is so
-        if not util.safehasattr(other, b'isascending'):
+        if not util.safehasattr(other, 'isascending'):
             # filter out hidden revision
             # (this boldly assumes all smartset are pure)
             #