I have built one that I call Mahout
(https://github.com/cbbrowne/mahout) which has the merit of involving
just two shell scripts, one of which is an auditing tool (pgcmp).
It implements a "little language" to indicate dependencies between the
SQL scripts that implement the DDL changes.
The notable thing that I have seen "downthread" that it does not
attempt to implement is "rollback scripts." I find that "necessity"
to be a ruby-on-rails dogma that does not seem to fit what I see
people doing.