Further improve handling of symlink targets/deps.

In commit redo-0.11-4-g34669fb, we changed os.stat into os.lstat to
avoid false positives in the "manual override" detector: a .do file
that generates $3 as a symlink would trigger manual override if the
*target* of that symlink ever changed, which is incorrect.

Unfortunately using os.lstat() leads to a different problem: if X
depends on Y and Y is a symlink to Z, then X would not be rebuilt when
Z changes, which is clearly wrong.

The fix is twofold:

1. read_stamp() should change on changes to both the link itself,
   *and* the target of the link.

2. We shouldn't mark a target as overridden under so many situations.
   We'll use *only* the primary mtime of the os.lstat(), not all the
   other bits in the stamp.

Step 2 fixes a few other false positives also.  For example, if you
'cp -a' a whole tree to another location, the st_ino of all the targets
will change, which would trigger a mass of "manual override" warnings.
Although a change in inode is sufficient to count an input as having
changed (just to be extra safe), it should *not* be considered a manual
override.  Now we can distinguish between the two.

Because the stamp format has changed, update the SCHEMA_VER field.  I
should have done this every other time I changed the stamp format, but
I forgot.  Sorry.  That leads to spurious "manually modified" warnings
after upgrading redo.

builder.py

@@ -147,7 +147,7 @@ class BuildJob:
         newstamp = sf.read_stamp()
         if (sf.is_generated and
             newstamp != state.STAMP_MISSING and 
-            (sf.stamp != newstamp or sf.is_override)):
+            (sf.is_override or state.detect_override(sf.stamp, newstamp))):
                 state.warn_override(_nice(t))
                 if not sf.is_override:
                     warn('%s - old: %r\n' % (_nice(t), sf.stamp))
@@ -345,7 +345,8 @@ class BuildJob:
             sf.is_override = False
             if sf.is_checked() or sf.is_changed():
                 # it got checked during the run; someone ran redo-stamp.
-                # update_stamp would call set_changed(); we don't want that
+                # update_stamp would call set_changed(); we don't want that,
+                # so only use read_stamp.
                 sf.stamp = sf.read_stamp()
             else:
                 sf.csum = None