using Undo modifies Bezier curves

[adrian17]

Steps:

1. Make a complex curve (using a Circle tool will do)
2. Modify one of the points (it's easiest to reproduce with big handle movements)
3. Click "undo".
   Some points that weren't modified at all get changed upon undoing.
   That doesn't happen 100% of the time, but it's easy to notice after a few tries.

Example:

[artursapek]

Thanks for the report. The history system is still a bit patchy. I'll take a look at it.