Detect false positive markers in root/parent directories.

For example, catalogue numbers like "[REACT217]". This shouldn't bypass the
nested multi-disc detection and automatically include all subdirs.

Do nested multi-disc detection first, so that `collapse_pat` is only set for
flattened albums, and we can skip the ancestry check on subsequent folders.
Merge pull request #91 from mrmachine/multi-disc

Detect false positive markers in root/parent directories.