cvsserver: checkout faster by sending files in a sensible order

cvsserver: checkout faster by sending files in a sensible order

Just by sending the files in an ordered fashion, clients can process them
much faster. And we can optimize our check of whether we created this
directory already -- faster.

Timings for a checkout on a commandline cvs client for a project with
~13K files totalling ~100MB:

Unsorted:
  603.12 real        16.89 user        42.88 sys

Sorted:
  298.19 real        26.37 user        42.42 sys