Skip over more lines in diffhunk selection

[danvk]

The diffhunk selection from #200 is functional but leaves a bit to be desired:

• Deleted diffhunks show a selection line both above and below the diffhunk (should only be above)
• When webdiff adds more context (e.g. to enforce minJumpSize), you can select the line where it did this. This looks weird and is non-sensical.
• Selection on top line is not displayed.
• It should be possible to select the bottom line.
• Hitting n at the bottom of the file should advance to the next file.
• Hitting p at the top of the file should move to the previous file.
• When you click an up/down arrow to show more context, the "seam" is still selectable.