Use git config ssh remote url to account for custom ssh aliases

[senyc]

I use an alias for my git remote url (so I can use separate ssh keys for my work and personal gh accounts). Currently the application will default to using ssh_url which is returned from the repos/{owner}/{repo}/pulls endpoint. This will always be git@github.com:<user>/<repo>.git.

In my case, and probably for others as well, this leads to the plugin being unable to run the git_fetch function, causing a failure on startup where the user would see failed to fetch remote branch in the upper-right hand corner, and be unable to do anything.

This addition will only do anything if the user is using the ssh git_protocol, if they are it will default the remote_url to whatever is configured in git, via git config --get remote.origin.url. Otherwise the old functionality will be used.