Fix bug #424 Wrong circular inclusion detection.
The current heuristic for detection of circular includes
fails with nested parsing. Line numbering differs in nested
state-machines, inclusions don't show up in the parent, if
a view is detached (e.g. for table cell content).
The new heuristic uses a "magic" comment instead of line numbers
to keep a log of recursive inclusions.
Add a test case for a false positive and a case for
too late detection in parsed includes.
Adapt tests to (irrelevant) side effects of the patch.
git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@8851 929543f6-e4f2-0310-98a6-ba3bd3dd1d04