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/state.py

@@ -1,5 +1,5 @@
 import sys, os, errno, stat, fcntl, sqlite3
-import env
+import cycles, env
 from helpers import unlink, close_on_exec, join
 from logs import warn, debug2, debug3
 
@@ -24,10 +24,6 @@ STAMP_MISSING = '0'   # the stamp of a nonexistent file
 LOG_LOCK_MAGIC = 0x10000000  # fid offset for "log locks"
 
 
-class CyclicDependencyError(Exception):
-    pass
-
-
 def _connect(dbfile):
     _db = sqlite3.connect(dbfile, timeout=TIMEOUT)
     _db.execute("pragma synchronous = off")
@@ -466,9 +462,7 @@ class Lock(object):
 
     def check(self):
         assert not self.owned
-        if str(self.fid) in env.get_locks():
-            # Lock already held by parent: cyclic dependence
-            raise CyclicDependencyError()
+        cycles.check(self.fid)
 
     def trylock(self):
         self.check()