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