This warning exists to prevent git users from prematurely polluting
their namespace when trying out Mercurial. But for repos that already
have multiple branches, understanding what branches are is not
optional so we should just shut up.
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.