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

@@ -65,7 +65,7 b' def encodevalueinheaders(value, header, '
 class _multifile:
     def __init__(self, *fileobjs):
         for f in fileobjs:
-            if not util.safehasattr(f, b'length'):
+            if not util.safehasattr(f, 'length'):
                 raise ValueError(
                     b'_multifile only supports file objects that '
                     b'have a length but this one does not:',