viewdiff: save memory by eliminating two captures

viewdiff: save memory by eliminating two captures

Avoid relying on $DIGIT captures when @- and @+ to access
last match start and end, respectively.  The elimination of
the post capture ought to allow the use of sv_chop to advance
the string start pointer without memory copies.

This ought to save 1-2MB of memory on my system since I've
noticed the captures was using a big chunk of scratchpad
space.