Some features do not work after GitLab version 13.5.
This update alerts the user that they are using an outdated
version.

Most of the client code was making its own `assert()`, because it
couldn't continue without having an existing repository. By moving the
assert into the `getRepository()` method, we simplify the client code.

Multiple paces will need to search through all the MR changes and find
the correct diff. This commit extracts a method to do that.

VS Code keeps the case. I.e. it  correctly replaces
even getWorkspaceFolders.

The only places where we didn't replace the string were when
we were using the VS Code API (vscode.workspace.workspaceFolders).

Extracting configuration related logic came up in reviews multiple times.
Also, the VS Code logic returns missing properties as null.
This behaviour is not ideal because null function arguments overrdie the
default function arguments.

Related discussions:
- https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/merge_requests/115#note_432956679