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

@@ -384,7 +384,7 b' def repo_rel_or_abs_source(repo):'
     Either absolute or relative the outermost repo"""
     parent = repo
     chunks = []
-    while util.safehasattr(parent, b'_subparent'):
+    while util.safehasattr(parent, '_subparent'):
         source = urlutil.url(parent._subsource)
         chunks.append(bytes(source))
         if source.isabs():