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/httppeer.py

@@ -180,7 +180,7 b' def makev1commandrequest('
     qs = b'?%s' % urlreq.urlencode(q)
     cu = b"%s%s" % (repobaseurl, qs)
     size = 0
-    if util.safehasattr(data, b'length'):
+    if util.safehasattr(data, 'length'):
         size = data.length
     elif data is not None:
         size = len(data)