Move env.{add,get}_lock() into cycles.py, and rename.

They really aren't locks at all, they're a cycle detector.  Also rename
REDO_LOCKS to a more meaningful REDO_CYCLES.  And we'll move the
CyclicDependencyError exception in here as well, instead of state.py
where it doesn't really belong.

redo/cycles.py

@@ -0,0 +1,24 @@
+"""Code for detecting and aborting on cyclic dependency loops."""
+import os
+
+
+class CyclicDependencyError(Exception):
+    pass
+
+
+def _get():
+    """Get the list of held cycle items."""
+    return os.environ.get('REDO_CYCLES', '').split(':')
+
+
+def add(fid):
+    """Add a lock to the list of held cycle items."""
+    items = set(_get())
+    items.add(str(fid))
+    os.environ['REDO_CYCLES'] = ':'.join(list(items))
+
+
+def check(fid):
+    if str(fid) in _get():
+        # Lock already held by parent: cyclic dependency
+        raise CyclicDependencyError()