Add `--history` flag to `dolt dump`

`dolt dump` currently generates a SQL script to recreate the database as it exists at the current branch head.

We should accept a `--history` flag to generate the entire history of the database. This means that it should emit SQL that reproduces every commit of the database, on every branch, with the same data, authors, timestamps, etc. This can be accomplished by emitting `call dolt_checkout()`, `call dolt_commit()` at various stages in the export process.