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/hgweb/webutil.py

@@ -211,7 +211,7 b' def _ctxsgen(context, ctxs):'
             b'description': s.description(),
             b'branch': s.branch(),
         }
-        if util.safehasattr(s, b'path'):
+        if util.safehasattr(s, 'path'):
             d[b'file'] = s.path()
         yield d