revlog: catch more specific exception in shortest()

Since revlog._partialmatch() catches RevlogError coming from cext and
re-raises AmbiguousPrefixLookupError, catching RevlogError here seems
less correct.

Differential Revision: https://phab.mercurial-scm.org/D4735

mercurial/revlog.py

@@ -1325,7 +1325,7 b' class revlog(object):'
         def isvalid(prefix):
             try:
                 node = self._partialmatch(prefix)
-            except error.RevlogError:
+            except error.AmbiguousPrefixLookupError:
                 return False
             except error.WdirUnsupported:
                 # single 'ff...' match