Look for changelog entries on the master branch.

The tag for the 3.16.1 release points to a commit whose last changelog entry is
about 3.16.0; the 3.16.1 changelog entry was added later on the master branch.
So this script spits out an error every time it runs:

  The last version in the release notes (3.16.0) is different from the newest tag (3.16.1+190225, version 3.16.1) at
  /var/lib/limesurvey/monitor-releases/git/bin/tails-monitor-limesurvey-releases line 11.

Let's support this corner case by looking for changelog entries on the master
branch, skipping versions newer than the latest upstream tag (to avoid asking
the service admins to upgrade to a version that's not released yet).