This PR adds `genpe.tja` to the test suite, a song that currently fails
due to how `tja2fumen` handles `#SECTION` commands.
We fix the problems with branching commands in
8ad2102f0a. (Making sure that any #SECTION
commands that occur before the first #BRANCHSTART are properly handled.)
This PR also contains a few refactoring commits to improve the clarity
of how the branching logic works, to make further debugging of branching
songs easier.
Fixes#33.
