This PR adds the remaining changes needed to support branching TJA
songs.
- Properly handling `r` (drumroll) based branching conditions
- Handling commands that come after a measure end but _before_ the first
`#BRANCHSTART` command. (This was not previously covered by
`hol6po.tja`.)
As well, it adds a bit of cleanup/refactoring to the changes from the
previous branch PR (#20).
To test these changes, another new branching TJA-fumen pair is added to
the test suite.
