linelog: be more careful about types before looking for _target attribute

Without this, pytype (correctly) complains that fakejge might not have
a _cmprev attribute. We're operating on some constraints not
expressible in the type system, so we detect the invalid case and
raise a nicer exception now.

Sadly, we also need an `assert` to give PyType a clue that we know
what we're doing, but I can absolutely live with that.

Differential Revision: https://phab.mercurial-scm.org/D7271

mercurial/linelog.py

@@ -278,8 +278,14 b' class linelog(object):'
         fakejge = _decodeone(buf, 0)
         if isinstance(fakejge, _jump):
             maxrev = 0
+        elif isinstance(fakejge, (_jge, _jl)):
+            maxrev = fakejge._cmprev
         else:
-            maxrev = fakejge._cmprev
+            raise LineLogError(
+                'Expected one of _jump, _jge, or _jl. Got %s.'
+                % type(fakejge).__name__
+            )
+        assert isinstance(fakejge, (_jump, _jge, _jl))  # help pytype
         numentries = fakejge._target
         if expected != numentries:
             raise LineLogError(