[JENKINS-19994] Dispose of RevWalk objects - avoid open files

Still have at least one case where the RevWalk object may be kept
open, and one case where a Git repository object may be kept open.
[JENKINS-19994] Close objects after ChangelogCommand.execute()

If the RevWalk is not closed, the JGit implementation leaves a pack
file open, which prevents the pack file from being deleted on Windows.

If the Repository is not closed, the JGit implementation may leave a
file open, preventing workspace deletion on Windows.