fix #151 asserting precondition
Merge pull request #151 from cordoval/151-the-pull-request-squash-dont-assert-preconditions

This PR was merged into gushphp:master branch.

Discussion
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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.

Commits
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d30412c01f4568c4542ca7890078e8b7705d0976 fix #151 asserting precondition cordoval