#61 Support for quick diff

[Johan Walles]

I'm running Eclipse 3.0 w. p4eclipse 0.6.24

Eclipse 3.0 has a Quick Diff feature that decorates the
leftmost column of the edit window with markers showing
which lines have been edited.

If I right click the left editor margin and choose "Set
Quick Diff Reference", my only choice is "Version on disk".

When I have p4eclipse installed and am editing a file
under p4's control, I should be able to choose "Head
revision" as well.