shortest: avoid magic number "41"

As suggested by Yuya. Feel free to fold into D3502.

Differential Revision: https://phab.mercurial-scm.org/D3531

mercurial/scmutil.py

@@ -466,7 +466,7 b' def shortesthexnodeidprefix(repo, node, '
     def disambiguate(prefix):
         """Disambiguate against revnums."""
         hexnode = hex(node)
-        for length in range(len(prefix), 41):
+        for length in range(len(prefix), len(hexnode) + 1):
             prefix = hexnode[:length]
             if not isrev(prefix):
                 return prefix