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/revlogutils/deltas.py

@@ -1067,7 +1067,7 b' class SnapshotCache:'
             end_rev < self._start_rev or end_rev > self._end_rev
         ), (self._start_rev, self._end_rev, start_rev, end_rev)
         cache = self.snapshots
-        if util.safehasattr(revlog.index, b'findsnapshots'):
+        if util.safehasattr(revlog.index, 'findsnapshots'):
             revlog.index.findsnapshots(cache, start_rev, end_rev)
         else:
             deltaparent = revlog.deltaparent