Credit: [@Musisimaru](https://gitlab.com/Musisimaru) (originally
    introduced in
    [!229](https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/merge_requests/229))

This is a copy paste of the code contributed by Mussimaru in
https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/merge_requests/229
This code doesn't compile, but I wanted to include it as a record that
he's done most of the work.

Credit: [@Musisimaru](https://gitlab.com/Musisimaru) (originally
introduced in
[!229](https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/merge_requests/229))

Credit: [@Musisimaru](https://gitlab.com/Musisimaru) (originally
introduced in
[!229](https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/merge_requests/229))

The issue was caused by passing down wrong arguments to the api fetch
method.

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This update alerts the user that they are using an outdated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

This was caused by converting the validation response to boolean instead
of returning all details.

By stipping away all but unchanged lines we allowed edge cases when
a file started or ended with changes. In this scenario we wrongly
addded unchanged lines to the beginning or the end of the file.

When we dispose commentController, all its therads will get disposed
automatically. We don't ever need to call dispose outside of the
GitLabCommentThread.

We can use the standard `getActiveRepositoryOrSelectOne()` and delete
the custom project picker.

Also, we can remove the duplicated block for getting project to put the
snippet in.

Lastly, this commit converts several nested if statement into early
return statements.

Creating snippets in user space hasn't worked in a long time. And for
the last year, when the extension supports multiple GitLab instances, we
wouldn't know which instance to use for adding the user snippet.

This commit removes the broken logic and focuses on creating project
snippets.

create_snippet was the only client using the function

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.

When we create new diff threads, the API requires us to submit all three
values (start/base/head SHA) so even though we don't need startSha
internally, we have to parse it from the response.