Fix sensitivity of prev/next diff in VC and DirDiff (closes bgo#650181)
Common code for finding the previous/next differing rows was moved to the tree module, and both VCView and DirDiff now use that approach. Previous/next diff is updated on tree cursor movement and used to set sensitivity of the corresponding actions. Currently there is no attempt to minimise the number of tree traversals done when the cursor moves. In some situations of very large trees with few changes, this may mean that scrolling through the tree with the keyboard will be very slow. There are certainly tricks that can be done with invalidation of the prev/next validity (TODO-ed in the code) that should improve the situation.
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