src/vs/workbench/api/common/extHostSCM.ts/ExtHostSourceControl/actionButton
class ExtHostSourceControl:	set actionButton(actionButton: vscode.SourceControlActionButton | undefined) {
		checkProposedApiEnabled(this._extension, 'scmActionButton');
		this._actionButtonDisposables.value = new DisposableStore();

		this._actionButton = actionButton;

		const internal = actionButton !== undefined ?
			{
				command: this._commands.converter.toInternal(actionButton.command, this._actionButtonDisposables.value),
				description: actionButton.description
			} : undefined;
		this.#proxy.$updateSourceControl(this.handle, { actionButton: internal ?? null });
	}