debugcommands: prevent using `is False`...
debugcommands: prevent using `is False`
I was touching this code in a future patch and marmoute warned about usage of
`is False` here.
Quoting marmoute:
```
"is False" is going to check if the object you have the very same object in
memory than the one Python allocated for False (in practice 0)
This will "mostly work" on cpython because of implementation details, but
is semantically wrong and can start breaking unexpectedly
```
Differential Revision:
https://phab.mercurial-scm.org/D10014
