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/debugcommands.py

@@ -1718,7 +1718,7 b' def debugformat(ui, repo, **opts):'
     if fm.isplain():
 
         def formatvalue(value):
-            if util.safehasattr(value, b'startswith'):
+            if util.safehasattr(value, 'startswith'):
                 return value
             if value:
                 return b'yes'