Discover the actual branch in Kokoro. (#108)
Discover the actual branch in Kokoro. (#108)

My initial approach to detect the branch in the Kokoro build was too
naive. Kokoro checks out the code in `detached HEAD` mode, that is,
the HEAD commit is not associated with any branch. And Kokoro does not
expose the branch name in an environment variable, as other CI systems
do. And the git version on Kokoro does not support
`git branch --short-current`.

In this PR we guess (or detect depending on how confident you feel) the
branch by looking at the current commit, finding out which branches
contain it, and then doing some filtering.
googleapis/google-cloud-cpp-spanner,googleapis/google-cloud-cpp-spanner,googleapis/google-cloud-cpp-spanner,googleapis/google-cloud-cpp-spanner