Get the branches each commit is on

A customer asked for a system table to list the branches each commit is on. You can do this with a recursive common table expression:

```
with recursive branch_commits (branch, commit) as (
select name, hash from dolt_branches
union
select branch, parent_hash
from dolt_commit_ancestors
join branch_commits
on commit = commit_hash
where parent_hash is not NULL
)
select * from branch_commits;
```

We should consider making this it's own system table, something like `dolt_commit_branches`.