"Open in Remote" commands should check for branch upstream tracking

[srenatus]

• GitLens Version: 6.1.0
• VSCode Version: 1.18.0
• OS Version: osx sierra

Steps to Reproduce:

1. for some repository, unset the upstream tracking of the checkout out branch: git branch --unset-upstream my/branch
2. for some file (path/to/foo.file) of that branch, click Open File in Remote

⏩ It will open https://github.com/org/repo/blob/fatal:%20no%20upstream%20configured%20for%20branch%20'my/branch'/path/to/foo.file#L203

It thus seems like the checks guarding Open File in Remote, gitlens:activeIsTracked && gitlens:activeHasRemotes are not enough: There also needs to be a check like activeIsTrackingUpstreamBranch -- alternatively, the exit code should be checked before opening the URL.

[srenatus]

@eamodio Thanks 😃

[github-actions]

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