Smartly clear the is-dirty flag if changes are undone.

With my work, I always have various files open for referencing (I use 2 column view mode so I keep these files on the left, with what I'm working on on the right).  Sometimes I accidentally type something into a reference file (because I didn't realize the cursor was in there), then press control-z to undo it.  The problem is now the reference file is marked as dirty.  Since these reference files are not checked out of P4, now every time I do a "save all" I always get a dialog telling me the file is not writable.  The only fix is to close the reference file tab and then re-open it.

Hence it would be swell if when you undo changes in the file up to the last save point, it would clear the is-dirty flag :)