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/patch.py

@@ -168,7 +168,7 b' def split(stream):'
 
     mimeheaders = [b'content-type']
 
-    if not util.safehasattr(stream, b'next'):
+    if not util.safehasattr(stream, 'next'):
         # http responses, for example, have readline but not next
         stream = fiter(stream)