view CONTRIBUTING @ 46467:45c3a263d5d1

engine: 'if not, else' -> 'if, else'

I personally feel that

```
if x:
    pass
else:
    pass
```

is easier to read and edit than

```
if not x:
    pass
else:
    pass
```

Next patches will add one more if-else clause.

Differential Revision: https://phab.mercurial-scm.org/D9931

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