OpenDiffOnSourcegraph links to compare page of revisions that are not available

Created by: felixfbecker

On Phabricator with staging repos, the two revisions being compared are not in the repo that contains the source. They are in a staging repo, that may not even be on the Sourcegraph instance.

The OpenDiffOnSourcegraph button currently links to the source repo on Sourcegraph, comparing the two revisions from the staging repo. It actually does an API request first that fails and logs an error, but then displays the link anyway. We should not show the button when that request fails.

However, it is also weird that in FileInfo we have rev and baseRev properties with a repoName property that doesn't match.