[NEW] undo changes to a file

[SRombautsU]

When a file is locally modified (or checked-out) it would be nice to be able to have a context menu action to undo the changes (and the checkout).

In git it's called "Discard Changes" for reference.

[nsxdavid]

In the Plastic client this seems to be called "undo". But yeah, so much missing in VS Code extension that is just basic.