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

@@ -1907,7 +1907,7 b' def debugindex(ui, repo, file_=None, **o'
 
     fm = ui.formatter(b'debugindex', opts)
 
-    revlog = getattr(store, b'_revlog', store)
+    revlog = getattr(store, '_revlog', store)
 
     return revlog_debug.debug_index(
         ui,