src/goLanguageServer: process only file, untitled doc schemes

As recommended in https://docs.microsoft.com/en-us/visualstudio/liveshare/reference/extensions#visual-studio-code-1
Even when gopls could handle vsls:/, that wouldn't be desirable -
the language servers in both host and guest machines would
work and produce duplicate results.

For #605

Change-Id: I953a81713be5e65a8b8e1597390b50a69ec4eed8
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/269157
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

src/goLanguageServer.ts

@@ -245,7 +245,19 @@ export async function buildLanguageClient(cfg: BuildLanguageClientOption): Promi
 		},
 		{
 			initializationOptions: goplsWorkspaceConfig,
-			documentSelector: ['go', 'go.mod', 'go.sum'],
+			documentSelector: [
+				// Filter out unsupported document types, e.g. vsls, git.
+				// https://docs.microsoft.com/en-us/visualstudio/liveshare/reference/extensions#visual-studio-code-1
+				//
+				// - files
+				{ language: 'go', scheme: 'file' },
+				{ language: 'go.mod', scheme: 'file' },
+				{ language: 'go.sum', scheme: 'file' },
+				// - unsaved files
+				{ language: 'go', scheme: 'untitled' },
+				{ language: 'go.mod', scheme: 'untitled' },
+				{ language: 'go.sum', scheme: 'untitled' },
+			],
 			uriConverters: {
 				// Apply file:/// scheme to all file paths.
 				code2Protocol: (uri: vscode.Uri): string =>