Add the ability to update the timestamp of the last commit

[acmiyaguchi]

The following situation currently requires manual intervention.

1. A modification is made to MPS that contains an incompatible change (fixing a mistake)
2. MSG generates a schema and pushes to MPS
3. Jenkins picks up on the new schema, enters a failure state

If the table is not dropped between steps 1 and 2, then Jenkins will need to rerun in order to update the table. In order to do this, there needs to be a new commit to the generated-schemas branch.

One solution to this is to allow git commit --amend to the last commit, in order to create a new commit hash with an updated timestamp. Every run of the MSG will then touch the timestamp of the schemas.