Add read_repository GitLab OAuth Scope

[ldennington]

Overview

Add read_repository to supported OAuth Scopes for GitLab. Because read_repository grants permissions to read files on private projects using the Repository Files API, it will enable Visual Studio to use Source Link to download files from private GitLab repositories 🎉. The Git Credential Manager GitLab OAuth Application has also been updated with the read_repository scope to work with this change.

🚨 Important Notes 🚨

In order to use the new read_repository scope, users will need to do the following with existing OAuth apps for their instances:

1. Update them to have the read_repository scope.
2. Require users to revoke the application by doing the following:
   1. Navigate to https://gitlab.com/-/profile/applications
   2. Scroll to Authorized applications
   3. Click the "Revoke" button next to the name of their instance's app (Git Credential Manager is used here for demonstration purposes)
      
   4. Wait for a notification stating The application was revoked access.
      
   5. Re-authorize the application with the new scope (GCM should handle this flow for them)

Additionally, although read_repository works for the API required by source link, our testing did uncover that there are certain APIs (listing a repository tree, for example) that require additional scopes (in this case, read_api). These scopes may also need to be added in the future.

Testing

I did the following to test these changes:

1. Create a new test OAuth application with write_repository and read_repository scopes.
2. Update GitLabDevClientId and GitLabDevClientSecret in my global Git config with the test Application ID and Secret.
3. Remove existing GitLab access/refresh tokens from Keychain.
4. Update this line to contain read_repository locally and build and install GCM with this new scope.
5. Clone a repo (to force re-authentication and add new access/refresh tokens to Keychain).
6. Copy the new access token from Keychain.
7. Attempt this CURL request.

curl "https://gitlab.com/api/v4/projects/36617582/repository/files/README.md/raw?access_token=<COPIED TOKEN>"

9. Verified contents of README.md were printed to my terminal.

Fixes #669

[ldennington]

@Bene81 as FYI.

[mjcheetham]

We may want to add the instructions on how to revoke access to the FAQs, or a link from an FAQ to a separate "how to revoke access on GitLab" markdown doc.

[hickford]

1. Update the OAuth app to have the read_repository scope.
2. Require users to revoke the application

Is step 2 necessary? What happens if the user doesn't revoke?

[ldennington]

Is step 2 necessary? What happens if the user doesn't revoke?

Based on my testing yesterday, if users do not manually revoke and re-authorize, the scopes for the application are not updated.