shortest: avoid magic number "41"

shortest: avoid magic number "41"

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

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

--- a/mercurial/scmutil.py	Fri May 11 07:31:51 2018 -0700
+++ b/mercurial/scmutil.py	Fri May 11 09:16:26 2018 -0700
@@ -466,7 +466,7 @@
     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