Undo crosses file boundaries

[LeifAndersen]

If I open some file A.cljs, and then immediately switch to file B.cljs, then press undo, the buffer switches back to A.cljs.

This is doubly wrong since the rest of the IDE show the information for B.cljs.

The buffers undo list should be cleared on file changes.

[LeifAndersen]

(Or better yet, kept in association with each file, so undo history can persist even after file switches.