[JENKINS-19994] Close files opened by computeChangeLog, use new abort() method

All calls to a changelog instance must conclude with either a call to
execute() or a call to abort().  Files will be left open if one of
those two methods is not called before the changelog instance goes out
of scope.
Merge pull request #185 from MarkEWaite/JENKINS-19994-close-changelog-opened-files

[JENKINS-19994] Close files opened by computeChangeLog, use abort()