To use changelog filtering on the repository, we plan to use "proxy" object that
perfectly mock a repository but with a filtered changelog.
Altering the `repo.commit` function using `extensions.wrapfunction` will prevent
the logic to propagate to the proxy class by the mean of inheritance.
We changes the extension to use subclassing as expectable.
Many tests didn't change back from subdirectories at the end of the tests ...
and they don't have to. The missing 'cd ..' could always be added when another
test case is added to the test file.
This change do that tests (99.5%) consistently end up in $TESTDIR where they
started, thus making it simpler to extend them or move them around.
