repair: fix an `isinstance(nodelist, str)` check for py3

All of the callers appear to pass a list, so this doesn't fix anything in core
hg.  But maybe out of tree extensions use this shortcut.

Differential Revision: https://phab.mercurial-scm.org/D7533

mercurial/repair.py

@@ -130,7 +130,7 b' def strip(ui, repo, nodelist, backup=Tru'
     cl = repo.changelog
 
     # TODO handle undo of merge sets
-    if isinstance(nodelist, str):
+    if isinstance(nodelist, bytes):
         nodelist = [nodelist]
     striplist = [cl.rev(node) for node in nodelist]
     striprev = min(striplist)