don't save sentinels in incremental history

This was doing nothing silently before, but the call to tuple(None) threw an
unhandled exception.
fix for the above fix
use unicode for import log

We pass all the paths involved through displayable_path now.
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.
tiny line wrap change
remove a redundant slash pointed out by @mrmachine