Make git-push-all.sh push branches in a specific order

git-push-all.sh currently pushes all the branches at once. Then GitHub sends requests to build each branch to Travis and Appveyor.

Depending on the exact order of the push and network requests, branches are built in an arbitrary order.

Ideally, we want to push branches in this order:

• master
  • practracker errors
  • errors that appear when merging backports to a later release
• maint branches, in order from the earliest release, to the most recent release
  • errors that appear when merging the PR into code that was added after the PR merge was build
• release branches, in the same order

So we should push branches in a for loop, with a sleep between each branch. On high-latency networks (me!), the sleep might need to be a few seconds long.