[JENKINS-19994] jgit close files

The JGit implementation leaves files open in the git repository.  That prevents deletion of the workspace as described in JENKINS-19994 and elsewhere.

In order to close the open files from the existing use cases for ChangelogCommand, the ChangelogCommand had to be extended to include an abort() method.  Either the abort() method or the execute() method must be called for each ChangelogCommand in order to close files opened by JGit.