scmutil: fix shortesthexnodeidprefix on Python 3 for 0-prefixed nodes

This fixes test-bookmarks.t on Python 3 (which had regressed.)

Differential Revision: https://phab.mercurial-scm.org/D3926

mercurial/scmutil.py

@@ -458,7 +458,7 b' def shortesthexnodeidprefix(repo, node, '
             # if we are a pure int, then starting with zero will not be
             # confused as a rev; or, obviously, if the int is larger
             # than the value of the tip rev
-            if prefix[0] == '0' or i > len(cl):
+            if prefix[0:1] == b'0' or i > len(cl):
                 return False
             return True
         except ValueError: