2.12.21 regtests

[Keith OHara]

Phooey. Between -20 and -21, full-measure rests are failing to get their space 
again.
   ‘multi-measure-rest-standard.ly'
   ‘multi-measure-rest-tweaks.ly’
   ‘spacing-measure-length.ly’

Maybe a side-effect of the changes to note-spacing (the 'extra-spacing-height 
rearrangements)
although I would /think/ that full-measure rests have no participation in 
note-spacing, rather just let the notes space themselves and then appear nicely 
centered in the resulting measure.  Probably something to do with how LilyPond 
ensures the measure is big enough for the rest.