Remove windows end of line from test case diff output

A ^M was being read into the diff output at the end of each line as
part of the test case.  That ^M caused test_show_revision_for_merge to
fail in the JGit implementation on Windows.
[JENKINS-19994] - JGit windows workspace won't delete

The JGit Repository object needs to be closed or it keeps files
open.  When files are open on Windows, the operating system refuses
to delete the files.
[JENKINS-19444] 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-19444] 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.