Presubmit flow

[vicb]

Currently we do test presubmit-* branches. If the test pass the branches are rebased on master and if the rebase went ok, they are merged into master.

Is see some drawbacks:

• We have to wait for the tests to finish before potentially failing a rebase,
• If something goes wrong during the rebase (unlikely to happen) we might have a master branch with broken code,
• Sometimes fixes are committed to the master branch and it would be nice to get them before executing the test suite (ie the recent dart2js workaround in tests).

What about the current workflow:

• Rebase presubmit-* branches before tests,
• Execute tests,
• Merge the branch into master

For the last part ("Merge the branch into master") it would be great to have a merge commit so that PRs could be reverted at once and to add GH PR discussion as notes to the commit - that would be nice further enhancements

[mhevery]

1. rename presubmit-* to branch-q-* where branch is the destination branch to rebase/merge to. i.e. master-q-pr-123
2. if branch-q-* then rebase onto branch before running the tests (in addition to after success)