Improve automerge script (#639)
Improve automerge script (#639)

- Now handles multiple PRs; moving on to the next when one gets stuck.
- Watches for the 'automerge' tag when invoked with no arguments
- Removes the tag when unable to merge without human help (e.g. merge conflicts)
- When a branch update triggered by the script causes googlebot to complain about CLAs (due to co-authorship), the script overrides the CLA check and deletes googlebot's comments
- Now works on PRs with compare branches from a fork
balopat/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,balopat/Cirq,quantumlib/Cirq