Added option for not writing tags to file, when tags in db are not modified when running beet modify
Synchronize with upstream beets modifications.
Removed dont_write_if_unchanged config option: now files tags are never updated unless db tags are changed (with beet modify)
Removed duplicated code: modified showdiff() so it uses also different()
Should infer *album artist* for albums from tag
Implement 117a331091111336c119bcb09c62dcdf23cae49f
avoid repeating diff check, use changed set (#511)

This way, _showdiff returns the information that _different used to provide
(since _showdiff needs to calculate it anyway). Using a `changed` set also
makes it easier to avoid unnecessary work.
changelog/thanks for #511
docs: fix reference to Transaction
Merge pull request #512 from geigerzaehler/infer_albumartist

Infer album artist from album artist tag
changelog/thanks for #512
Refactor import test
Extract ImportHelper for tests
Extract _run_import and expose config in tests
Use _setup_library() helper
Extract library file assertions
Make test strings more verbose
So this is how python works! ;)
Move prune and file tasks
These tests are already in ImportNonAutotaggedTest