Switch tab removes undo ability

[octref]

Make some changes in one file, switch to another file, then switch back. You can't undo with u, but Cmd+z still works.

• VSCode Version: 1.7 Insider
• VsCodeVim Version: 0.3.8
• OS: OS X 10.11.6

[rebornix]

@octref for your information, here is the root cause microsoft/vscode#14118

[octref]

Thanks!