revlog: avoid hard-coded hash sizes

Differential Revision: https://phab.mercurial-scm.org/D8683

mercurial/revlog.py

@@ -1523,7 +1523,7 b' class revlog(object):'
 
         def disambiguate(hexnode, minlength):
             """Disambiguate against wdirid."""
-            for length in range(minlength, 41):
+            for length in range(minlength, len(hexnode) + 1):
                 prefix = hexnode[:length]
                 if not maybewdir(prefix):
                     return prefix
@@ -1540,12 +1540,12 b' class revlog(object):'
                 pass
 
         if node == wdirid:
-            for length in range(minlength, 41):
+            for length in range(minlength, len(hexnode) + 1):
                 prefix = hexnode[:length]
                 if isvalid(prefix):
                     return prefix
 
-        for length in range(minlength, 41):
+        for length in range(minlength, len(hexnode) + 1):
             prefix = hexnode[:length]
             if isvalid(prefix):
                 return disambiguate(hexnode, length)