Test extension provided read-only message

[alexr00]

Refs: #166971

• anyOS @daviddossett
• anyOS @hediet

Complexity: 2

Create Issue

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We have a new API proposal for extension that have a read-only file system provider to provide markdown for the message that is shown when the user ties to edit a read-only file:

vscode/src/vscode-dts/vscode.proposed.readonlyMessage.d.ts

Lines 12 to 13 in 2e9aae6

	export function registerFileSystemProvider(scheme: string, provider: FileSystemProvider, options?: { readonly isCaseSensitive?: boolean; readonly isReadonly?: boolean | MarkdownString }): Disposable;
	}

The GitHub Pull Requests and Issues extension uses the this proposal to show a useful message if you try to edit a file in a PR that isn't currently checked out.

To verify:

1. Install the pre-release version of GitHub Pull Requests and issues
2. Open a folder that contains a clone of a GitHub repo.
3. From the "Pull Requests" view, expand a PR and open a file diff
4. Try to edit the diff
5. Verify that you see a message that instructs you to check out the PR to edit.
6. Verify that you can click on the "Check out" in the message to check out the PR.

[daviddossett]

Works great! Just a few nits.