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.
