Selection: Set context for precompiled leading-ID selectors

Might as well optimize `#id …` for precompiled selectors as well.

I recommend browsing the diff with [?w=1](?w=1) to suppress noise from the indentation change.

```
   raw     gz Compared to master @ f83f57d5eaf881015ad9b1a45903f9ed670cd626
   +56    +44 dist/sizzle.js
    +9     +2 dist/sizzle.min.js
```