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/subrepoutil.py

@@ -428,7 +428,7 b' def _abssource(repo, push=False, abort=T'
             return bytes(parent)
     else:  # recursion reached top repo
         path = None
-        if util.safehasattr(repo, b'_subtoppath'):
+        if util.safehasattr(repo, '_subtoppath'):
             path = repo._subtoppath
         elif push and repo.ui.config(b'paths', b'default-push'):
             path = repo.ui.config(b'paths', b'default-push')