gitserver: fetch GitLab and Bitbucket pull refs

Created by: keegancsmith

We have EnsureRevision logic which will trigger a fetch if a revision is missing. This will run when browsing a pull request on GitLab. However, if the pull request is not from a branch in the same repository our fetch would not fetch the relevant commit. This leads to us constantly fetching and failing.

In particular we are noticing "git fetch" constantly running for gitlab.com/gitlab-org/gitlab and in some cases it running very slowly. I assume this gitlab rate limiting/metering our requests. The metering leads to an unresponsive Sourcegraph request since we wait upto 60s for the fetch to happen for a missing revision.

Fixes https://github.com/sourcegraph/sourcegraph/issues/10960