Half-support for using file checksums instead of stamps.

A new redo-stamp program takes whatever you give it as stdin and uses it to
calculate a checksum for the current target.  If that checksum is the same
as last time, then we consider the target to be unchanged, and we set
checked_runid and stamp, but leave changed_runid alone.  That will make
future callers of redo-ifchange see this target as unmodified.

However, this is only "half" support because by the time we run the .do
script that calls redo-stamp, it's too late; the caller is a dependant of
the stamped program, which is already being rebuilt, even if redo-stamp
turns out to say that this target is unchanged.

The other half is coming up.

redo-stamp.py

@@ -0,0 +1,43 @@
+#!/usr/bin/python
+import sys, os
+import vars, state
+from helpers import err, debug2
+
+if len(sys.argv) > 1:
+    err('%s: no arguments expected.\n' % sys.argv[0])
+    sys.exit(1)
+
+# hashlib is only available in python 2.5 or higher, but the 'sha' module
+# produces a DeprecationWarning in python 2.6 or higher.  We want to support
+# python 2.4 and above without any stupid warnings, so let's try using hashlib
+# first, and downgrade if it fails.
+try:
+    import hashlib
+except ImportError:
+    import sha
+    sh = sha.sha()
+else:
+    sh = hashlib.sha1()
+
+while 1:
+    b = os.read(0, 4096)
+    sh.update(b)
+    if not b: break
+
+f = state.File(name=vars.TARGET)
+csum = sh.hexdigest()
+changed = (csum != f.csum)
+debug2('%s: old = %s\n' % (f.name, f.csum))
+debug2('%s: sum = %s (%s)\n' % (f.name, csum,
+                                changed and 'changed' or 'unchanged'))
+f.is_generated = True
+f.is_override = False
+f.failed_runid = None
+if changed:
+    f.set_changed()  # update_stamp might not do this if the mtime is identical
+    f.csum = csum
+else:
+    # unchanged
+    f.set_checked()
+f.save()
+state.commit()