shortest: fix an off-by-1 when disambiguating against revnums

Revision "5" is an unambiguous hex prefix in a repo with 5 revisions;
it's only "4" and lower that are ambiguous.

Differential Revision: https://phab.mercurial-scm.org/D4420

mercurial/scmutil.py

@@ -475,7 +475,7 b' def mayberevnum(repo, prefix):'
         # 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[0:1] == b'0' or i > len(repo):
+        if prefix[0:1] == b'0' or i >= len(repo):
             return False
         return True
     except ValueError: