Out-of-band rebases can lead to bad restacks

[mattprecious]

We keep track of parent SHAs in the branch table and use them when restacking. Performing rebases/restacks out-of-band can cause these SHAs to become stale, which can result in commits being dropped or mangled when performing a subsequent restack using stacker.

[mattprecious]

One that just got me:

st branch create change-a
// Make changes.
git commit
// Make more changes
git commit
st branch create change-b
git checkout change-a
git reset --hard HEAD~1
st upstack restack
// Second commit is gone from change-b