Don't re-check dependencies in a single run.

If a depends on b depends on c, then if when we consider building a, we have
to check b and c.  If we then are asked about a2 which depends on b, there
is no reason to re-check b and its dependencies; we already know it's done.

This takes the time to do 'redo t/curse/all' the *second* time down from
1.0s to 0.13s.  (make can still do it in 0.07s.)

'redo t/curse/all' the first time is down from 5.4s to to 4.6s.  With -j4,
from 3.0s to 2.5s.

state.py

@@ -34,6 +34,7 @@ def _stampname(t, fromdir=None):
     
 
 def stamp(t):
+    mark(t)
     stampfile = _stampname(t)
     newstampfile = _sname('stamp' + str(os.getpid()), t)
     depfile = _sname('dep', t)
@@ -65,6 +66,25 @@ def stamped(t, fromdir=None):
     return stamptime
 
 
+def mark(t, fromdir=None):
+    try:
+        open(_sname('mark', t, fromdir), 'w').close()
+    except IOError, e:
+        if e.errno == errno.ENOENT:
+            pass  # may happen if someone deletes our .redo dir
+        else:
+            raise
+
+
+_marks = {}
+def ismarked(t, fromdir=None):
+    if _marks.get((t,fromdir)):
+        return True
+    if os.path.exists(_sname('mark', t, fromdir)):
+        _marks[(t,fromdir)] = True
+        return True
+
+
 def is_generated(t):
     return os.path.exists(_sname('gen', t))