Support merging pull requests

[dan-zheng]

This involves configuring Copybara properly.

Should be ready soon!

[dan-zheng]

Testing workflow for merging pull requests on #14.

[dan-zheng]

Our workflow is working reasonably well now.

For maintainers:

• Change pull requests to retarget the google:dev branch, if not already the target.
• Add the copybara-import label on the pull request and wait for Copybara to import the pull request internally.
• Approve the imported changelist.
  • Eventually, the Copybara service should automatically merge the pull request as well.
  • In the short-term, this is bugged, so manually merge the pull request instead (with a squash commit).