Issue 5584: Rehabilitate regtest workflow (issue 571030043 by address@hidden)

[nine . fierce . ballads]

Reviewers: ,

Description:
https://sourceforge.net/p/testlilyissues/issues/5584/

0. fix repeated test runs for input/regression/midi

   this is issue 5582 but other changes in this review depend on it

1. fix multiple rules for target collated-files.list

2. don't create the output dir just to remove it

3. include count of added/removed regtests in summary

4. sort changed.txt and ensure a final newline

5. rehabilitate regtest workflow

   "make test-baseline" now incorporates "make test-clean"

   "make check" now works as make test-redo was supposed to:
   changed test cases are re-tested

6. Contributor's Guide updates

Please review this at https://codereview.appspot.com/571030043/

Affected files (+82, -57 lines):
  M Documentation/contributor/programming-work.itexi
  M Documentation/contributor/regressions.itexi
  M GNUmakefile.in
  M input/regression/lilypond-book/GNUmakefile
  M input/regression/midi/GNUmakefile
  M make/lysdoc-rules.make
  M make/lysdoc-targets.make
  M make/lysdoc-vars.make
  M make/midi-rules.make
  M make/midi-vars.make
  M scripts/build/output-distance.py
  M stepmake/stepmake/generic-targets.make
  M stepmake/stepmake/texinfo-rules.make
  M stepmake/stepmake/toplevel-targets.make