Don't crash on cyclic references between rule bindings.

Don't crash on cyclic references between rule bindings.

Fixes #902.

This dynamically detects cycles.  I like this approach less than
detecting them statically when parsing rules [1], but it has the
advantage that it doesn't break existing ninja manifest files.

It has the disadvantage that it slows down manifest_parser_perftest by
3.9%.

1: https://github.com/martine/ninja/commit/cc6f54d6d436047