We can save a lot (90%) of execution time on some repos with two
changes:

 - Not recursing into directories that are a priori excluded from the
   formatter's list (e.g., hidden directories like .git). Note that this
   means we no longer build and log a comprehensive list of excluded
   files. I think it's worth it to run so much faster.

 - Removing the collapseDirectories feature, to be replaced with
   batch parallel invocations. Computing the collapsible directories
   was some kind of superlinear deduplication. This could maybe be
   optimized in a way that would allow it to remain, but it seems it was
   intended as a stopgap solution for argv limitations in the first
   place.