A tab before an opening ``` panics doc tests

[Stebalien]

When a tab appears between /// and ``` in documentation code examples:

///<tab>```
///<tab>println!("here");
///<tab>```

The doc tests fail with:

<stdin>:2:5: 2:6 error: unknown start of token: `
<stdin>:2     ```
              ^
thread '<unnamed>' panicked at 'Box<Any>', ../src/libsyntax/parse/lexer/mod.rs:167

The worst part is that this tab looks like a space when tabwidth is set to 4.

[Stebalien]

Note: this doesn't happen when preceded by other doc text:

///<tab>a
///<tab>```
///<tab>println!("here");
///<tab>```

[mitaa]

This is fixed