Fix command palette sign in command

[RMacfarlane]

Fixes #1213

The pr.signin command was previously reading a property that hadn't been initialized. Changed to read from remotes directly instead.

[rebornix]

The logic is good. My only question is whether we should check if this._githubRepositories exits or not already when running login as checking all remotes is probably costly (every authority is one API call to the server).

[RMacfarlane]

Since this._githubRepositories is initialized to an empty array, it's hard right now to check if its already up to date. And since sign in should be run infrequently, I think it's ok to make the expensive call for now