src/eval.c: Fix bug#62419

src/eval.c: Fix bug#62419

Yup, almost 40 years after ELisp first combined them, buffer-local
and let bindings still don't work quite right :-(

The "automatically buffer-local if set" semantics should follow the
principle that it becomes buffer-local iff the var's current binding
refers to the top-level/global/non-let binding.

* src/eval.c (let_shadows_buffer_binding_p): Disregard non-global
let-bindings.

* test/src/eval-tests.el (eval-test--bug62419): New test.