Issue 5584: Rehabilitate regtest workflow

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

   the approach is to move the ly files that are the result of
   converting ly -> midi -> ly into a subdirectory to keep
   them separate from the ly files produced by lilypond-book

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

[lemzwerg, 2019-10-23 17:50:55]

LGTM, thanks!

[Dan Eble, 2019-11-04 18:03:31]

https://codereview.appspot.com/571030043/diff/579010043/scripts/build/output-...
File scripts/build/output-distance.py (right):

https://codereview.appspot.com/571030043/diff/579010043/scripts/build/output-...
scripts/build/output-distance.py:1050: return make_row (label, value) if value
else ''
It just occurred to me to test with Python 2.4 before pushing, and it caught
this.  I'll change it to multi-line syntax and test on my own before pushing.

[hahnjo, 2019-11-04 18:20:27]

On 2019/11/04 18:03:31, Dan Eble wrote:
>
https://codereview.appspot.com/571030043/diff/579010043/scripts/build/output-...
> File scripts/build/output-distance.py (right):
> 
>
https://codereview.appspot.com/571030043/diff/579010043/scripts/build/output-...
> scripts/build/output-distance.py:1050: return make_row (label, value) if value
> else ''
> It just occurred to me to test with Python 2.4 before pushing, and it caught
> this.  I'll change it to multi-line syntax and test on my own before pushing.

Sorry, I wanted to test but forgot until now :-( let me know if you wanted a
second pair of eyes, I'm working on LilyPond right now anyway.

[Dan Eble, 2019-11-04 18:40:45]

On 2019/11/04 18:20:27, hahnjo wrote:
> Sorry, I wanted to test but forgot until now :-( let me know if you wanted a
> second pair of eyes, I'm working on LilyPond right now anyway.

No need to apologize.  Anyway, it probably would have been a challenge to apply
this patch to master until a few hours ago when the revised changes for Issue
5578 made it back in.