Before this patch, release_seen_and_tovisit did not return NULL, so the
exception was not raised immediately. As Py_XDECREF() and free() are safe
for NULL, we can simply bail in any case.
If revlog file is corrupted, it can have parent pointing to invalid revision.
So we should validate it before updating nothead[], phases[], seen[], etc.
Otherwise it would cause segfault at best.
We could use "rev" instead of "maxrev" as upper bound, but I think the explicit
"maxrev" can clarify that we just want to avoid possible buffer overflow
vulnerability.
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.
