Certain redo post-build failures would still mark a target as built.

If we failed because:
- target dir doesn't exist
- failed to copy from stdout
- failed to rename $3

We would correctly return error 209, but the target would still be
marked as having been built, so redo-ifchange would not try to build it
next time, beause we forgot to call sf.set_failed() in those cases.

minimal/do worked correctly.

Added a test to catch this in the future.

redo/builder.py

@@ -419,7 +419,8 @@ class _BuildJob(object):
                 sf.csum = None
                 sf.update_stamp()
                 sf.set_changed()
-        else:
+        # rv might have changed up above
+        if rv:
             helpers.unlink(self.tmpname)
             sf = self.sf
             sf.set_failed()