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.

redo-ifchange.py

@@ -5,6 +5,8 @@ from helpers import debug, err, mkdirp, unlink
 
 
 def _dirty_deps(t, depth, fromdir):
+    if state.ismarked(t, fromdir):
+        return False  # has already been checked during this session
     debug('%s?%s\n' % (depth, t))
     stamptime = state.stamped(t, fromdir)
     if stamptime == None:
@@ -29,6 +31,7 @@ def _dirty_deps(t, depth, fromdir):
             if dirty_deps(name, depth + '  ', fromdir=vars.BASE):
                 #debug('%s-- DIRTY (sub)\n' % depth)
                 return True
+    state.mark(t, fromdir)
     return False