Be able to run a test from a different branch without doing checkout

[raulb]

Currently, we're only running tests in a current branch. i.e. If you wanted to run a test in a branch different than master, we require doing git checkout before.

It'd be nice if we could do the same without requiring that step.

e.g.:

Now:

$ git checkout branch-name
$ heroku ci:test

Proposal:

$ heroku ci:test --branch branch-name