[JENKINS-19444] Dispose of RevWalk objects at method exit - avoid keeping files open

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-19444] Close RevWalk and Repository on exit from 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.