DOLT_CHECKOUT('-b', 'newbranch) should take working set changes to new branch

[VinaiRachakonda]

For example doing DOLT_MERGE('--squash') leaves the state with a dirty working set. We should be able to do a DOLT_CHECKOUT from there.

[zachmu]

There's currently no way to stash session changes before changing branches. This would let customers move them to a new branch and commit them there.

[timsehn]

We decided that in the SQL context DOLT_CHECKOUT() does not take working set changes.