feat: adds script for git migration.

[atulep]

Script that migrates Git history of a split repo to google-cloud-python.

It copies Git history from source repo (i.e. "split repo") to target repo (i.e. google-cloud-python). Then, creates a pull request in target repo.

[vchudnov-g]

One relatively minor comment.

But I do think we want the other open comment, deriving ${ARTIFACT_NAME} from the split repo.

[vchudnov-g]

Looks good. Only issue is about the commented-out gh command. Could you address that before merging?

[vchudnov-g]

As per offline chat, sounds like we also need to mkdir -p owl-bot-staging/${DISTRIBUTION_NAME} after validation and before creating the PR.

Which means maybe we do the jq extract into ${DISTRIBUTION_NAME} and then set TARGET_PATH="packages/${DISTRIBUTION_NAME}" above?

[vchudnov-g]

As per correction in chat, we'll need to

mkdir -p owl-bot-staging/${DISTRIBUTION_NAME}/${DISTRIBUTION_NAME}
touch owl-bot-staging/${DISTRIBUTION_NAME}/${DISTRIBUTION_NAME}/${DISTRIBUTION_NAME}.txt
git add owl-bot-staging
git commit -m "Trigger owlbot post-processor"

although it's not clear this is sufficient to make the migrated repo pass pre-submits.

[atulep]

Ok, added owl-bot-staging. But, we should add any necessary fixes, if needed, in follow-up PRs.

[vchudnov-g]

One non-trivial comment about making the title have a conventional commit format.