Make Copybara close pull requests upon merge, use more informative commit descriptions

[dan-zheng]

1. Copybara does not merge or close pull requests upon importing commits to dev branch. This makes it unclear when pull requests have truly been "merged" and requires us to manually close PRs.
   • It would be great if Copybara automatically closed pull requests upon merge instead.
2. Copybara-exported commits start with Copybara import of the project: in the first line, which provides no information.
   • It would be good to show pull request descriptions in the first line instead.

[dan-zheng]

This seems to be working now, looking at #33 as an example.

Copybara produces a merge commit: 1a1dd90

And faithfully merges commits from the pull request, preserving commit descriptions:

This is an improvement over the old Copybara import of the project: commit descriptions!