Currently, it's not possible to change a SourceControlResourceGroup's label once it's created. From vscode.d.ts:

export interface SourceControlResourceGroup {
		readonly id: string;
		readonly label: string; 
                .......

In Perforce, a changelist has a description which can be edited by the user. To refresh that one changelist's description, an extension would have to do changelist.dispose() the existing changelist, and call SourceControl.createResourceGroup() again with the new description. This has a major side effect, because resource groups are re-arranged in the scm viewlet once an existing one is dispose()'d.

This PR makes it possible to change a SourceControlResourceGroup's label without having to call dispose() and createResourceGroup().