Re: [PATCH v4 01/21] travis: fix skipping tagged releases

"Johannes Schindelin via GitGitGadget" [off-list ref]
writes:

From: Johannes Schindelin <redacted>

When building a PR, TRAVIS_BRANCH refers to the *target branch*.
Therefore, if a PR targets `master`, and `master` happened to be tagged,
we skipped the build by mistake.

Fix this by using TRAVIS_PULL_REQUEST_BRANCH (i.e. the *source branch*)
when available, falling back to TRAVIS_BRANCH (i.e. for CI builds, also
known as "push builds").

Let's give it a new variable name, too: CI_BRANCH (as it is different
from TRAVIS_BRANCH). This also prepares for the upcoming patches which
will make our ci/* code a bit more independent from Travis and open it
to other CI systems (in particular to Azure Pipelines).

Makes sense.