gitlab: use baseURL to match the correct OAuth context
Created by: unknwon
Previously in https://github.com/sourcegraph/sourcegraph/pull/39840, we would always pick the first GitLab auth provider to obtain the OAuth context, which isn't a problem when there is only one GitLab auth provider configured (in most of the cases). However, this would give us incorrect OAuth context when there are more than one GitLab auth providers configured.
This PR fixes that by using the root URL for determining the correct auth provider to get OAuth context.
Test plan
Unit tests
Followup of https://github.com/sourcegraph/sourcegraph/pull/39840#discussion_r951759885