Fix a deadlock with redo-oob.

If a checksummed target A used to exist but is now missing, and we tried to
redo-ifchange that exact file, we would unnecessarily run 'redo-oob A A';
that is, we have to build A in order to determine if A needs to be built.

The sub-targets of redo-oob aren't run with REDO_UNLOCKED, so this would
deadlock instantly.

Add an assertion to redo-oob to ensure we never try to redo-ifchange the
primary target (thus converting the deadlock into an exception).  And skip
doing redo-oob when the target is already the same as the thing we have to
check.

redo-oob.py

@@ -10,6 +10,9 @@ if len(sys.argv[1:]) < 2:
 target = sys.argv[1]
 deps = sys.argv[2:]
 
+for d in deps:
+    assert(d != target)
+
 me = state.File(name=target)
 
 os.environ['REDO_NO_OOB'] = '1'