mergeview/wiggle: improvements for space-only conflicts.

mergeview/wiggle: improvements for space-only conflicts.

1/ change how the "wiggle" is reported.  It is no longer reported
   by "set-wiggle".  Instead a new "get-result" function returns
   various results including the "wiggle".
2/ count how many conflicts only involve white space.  If that is
   all the conflicts, then still calculate the wiggle.
3/ If a merge only has space conflicts, use blue for the merge
   markers rather the red (or green).
4/ mark the green merge markers bold so they stand out more
5/ When adding attrs, annotate conflicts which are space only,
   and respond to that with underlines instead of inverse red.

Signed-off-by: NeilBrown <neil@brown.name>