Move check_changelog_sha1 earlier in the file - define before call
Do not overwrite def of "w" with a different type of "w"

The "w" in the file is a private field used to access the working
area.  The "w" in the modified method was replacing that class private
field with a method variable also named "w", but used as a
StringWriter.  Poor variable naming on my part.
Annotate test_changelog as not implemented in JGit, even though it is

The changelog() method returns no content from JGit, even though the
same method from CGit returns the expected content.  Rather than allow
that significant difference in behavior to distract me from other bug
fixing work, I'm leaving this test as only executed on CGit.
Add trivial test of SHA1 bounded changelog

Works for both CGit and JGit, which hints that this is probably the
method used in Jenkins.