In I77a5b606 we attempted to limit the topo sort using JGit's merge
base calculation, but this requires one RevFlag per start commit,
which fails if there are more than 26 related changes in a list--
uncommon, but supported.
That approach also had some other drawbacks, so just throw it out.
Instead, do a partial topo sort manually, which ensures that no parent
is emitted before a direct child also in the list.
Change-Id: I7e87b3fc6885e319e5ca9b9bb974f65e016c647d
c212321f20