The `refs` variable is used as parameter to indicate which commits
to checkout, but it also was used as result variable to store the
ids of the commits that actually got checked out; naturally, it
was initialized to the empty array. This of course meant nothing
was ever actually checked out. Doh.
New input type that takes a ostree repo and checks out any number
of commits inside that repo to a temporary directory. Each commit
will be checked out to a separate sub-directory. The name of the
dir is the commit id of the corresponding commit.
This input can thus be used to access files and directories of
commits in stages.