The "pull-request:squash" don't assert preconditions

Here is what I do:

* I'm the owner of the repo
* I'm running `pull-request:list` to see all pull requests to my repo
* I discover pull request sent by other contributor
* issue `pull-request:squash` command
* command fails because I don't have push access to contributor repo

I think that before doing anything we must verify push access to the repo we're trying to squash.

I suppose that `pull-request:squash` command was meant for my own PRs.