[pkg_deb] Fix multiline fields in changes file

[Homulvas]

Fix broken .changes file reported in #632
As per documentation https://www.debian.org/doc/debian-policy/ch-controlfields.html#debian-changes-files-changes
Files and checksum fields always start with an empty line.
Fix this by marking the fields as is_multiline=True. Also remove the trailing whitespace after a single line includes just the field key. Added a test that checks for an exact match to generated .changes file.

fixes #632
fixes #659

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[Homulvas]

Since I hardcoded the checksums in the test case the tests now show that on macos the resulting deb has a different checksum and the date is off by one hour in the changes file. I think ideally this rule should produce an identical package across different environments.

[aiuto]

So then the question is if that is correct or not. IRRC, there were another set of bugs about the control file fields being wrong. I don't recall if they were on github or internal.
Do you have pointers to the definitive specs. I'm going to be tied up for a while and won't have time to look deeply into that before end of April.

[Homulvas]

https://www.debian.org/doc/debian-policy/ch-controlfields.html#description describes how Description is handled differently between control and changes files.
If you grep is always empty in the doc you'll find that it mentions Files, Changes, and Checksums-x these are the fields that were changed to be multiline in this PR (these fields are not part of the control file anyway).

[phlax]

i would suggest using dscverify to validate the package files

[Homulvas]

i would suggest using dscverify to validate the package files

before

$ dscverify fizzbuzz_4.5.6_all.changes --no-sig-check
fizzbuzz_4.5.6_all.changes:
dscverify: no file spec lines in fizzbuzz_4.5.6_all.changes
Validation FAILED!!
$ changestool fizzbuzz_4.5.6_all.changes verify
Checking Source packages...
Checking Binary consistency...
ERROR: 'fizzbuzz_4.5.6_all.changes' has binary 'fizzbuzz' in 'Binary:' and 'Description:' header, but no files for it found!
Checking checksums...

after

$ dscverify fizzbuzz_4.5.6_all.changes --no-sig-check
fizzbuzz_4.5.6_all.changes:
   validating fizzbuzz_4.5.6_all.deb
All files validated successfully.
$ changestool fizzbuzz_4.5.6_all.changes verify
Checking Source packages...
Checking Binary consistency...
Checking checksums...

[Homulvas]

@aiuto can you land this as I don't have access to that?