revlog: add a fast path for "ambiguous identifier"

Before fd1bb7c, if the C index.partialmatch raises RevlogError, the Python
code raises "ambiguous identifier" error immediately, which is efficient.

fd1bb7c took hidden revisions into consideration and forced the slow path
enumerating the changelog to double-check hidden revisions. But it's not
necessary if we know the revlog has no hidden revisions.

This patch adds back the fast path for unfiltered revlogs.

mercurial/revlog.py

@@ -941,8 +941,11 b' class revlog(object):'
             return None
         except RevlogError:
             # parsers.c radix tree lookup gave multiple matches
+            # fast path: for unfiltered changelog, radix tree is accurate
+            if not getattr(self, 'filteredrevs', None):
+                raise LookupError(id, self.indexfile,
+                                  _('ambiguous identifier'))
             # fall through to slow path that filters hidden revisions
-            pass
         except (AttributeError, ValueError):
             # we are pure python, or key was too short to search radix tree
             pass