Undoing while switching from serving one user to the next does not return to the previous user.

[nus-pe-bot]

When switching between different served users in Serve mode, using the serve mode command serve K\0001, then serve K\0002, then undo(a normal mode command) does not "undo" the serving of K\0002. instead, serve mode is exited

---

_{[original: nus-cs2103-AY1920S1/ped-interim#3049]}