Tentative fix for issue 2734: also check for RETAG

[Vrihub]

This is a tentative fix for issue #2734 , based on the suggestion in #2734 (comment)

I've tested it and it fixes the problem for me, but I'm not sure if other changes are required for a complete solution, since my knowledge of the code base is very limited. Please review!

[wisp3rwind]

Yep, that's exactly the code I meant to suggest.

grep -r RETAG shows that the edit plugin is the only piece of code using this choice and the following comment from ìmporter.py also suggests that the difference between APPLY and RETAG is really only the source of the metadata. Apart from that they both occur for the same kinds of import tasks and the same plugin actions should probably happen.

action = Enum('action',
              ['SKIP', 'ASIS', 'TRACKS', 'APPLY', 'ALBUMS', 'RETAG'])
# The RETAG action represents "don't apply any match, but do record
# new metadata". It's not reachable via the standard command prompt but
# can be used by plugins.

Note that I'm not intimately familiar with the importer, either.

[sampsyo]

This looks great; thank you!

It looks like Travis has pointed out a too-long line: https://travis-ci.org/beetbox/beets/jobs/297512858#L1095

Also, would you mind adding a changelog entry that summarizes the problem that was fixed?

[Vrihub]

Uhm, no idea why AppVeyor fails here: https://ci.appveyor.com/project/beetbox/beets/build/2228

[sampsyo]

Thank you! ✨ I'm pretty sure that AppVeyor failure is spurious, so I'll merge this now. Woohoo!