Progress location for comments

The GitHub Pull Requests and Issues extension uses the comments API to create GitHub PR comments. It does this in the following way: 1. Offers a commenting range. 3. When the + in the range is used, VS Code prompts for a comment. 4. User enters a comment and uses the extension provided "Start review" or "Add a Comment" button. 5. Extension is now responsible for adding the new comment to the new comment thread. At no point here is there an opportunity for the extension to show progress on the comment or comment thread. In GHPRI's case, it can take a second to complete the GitHub API call that actually adds the comment to the review. Currently, we add a fake comment to prevent a flicker, but that can be misleading for users (see https://github.com/microsoft/vscode-pull-request-github/issues/4291 which sparked this new issue).