On systems where 'python' refers to python3, redo
failed to launch. All invocations of python have been
made explicitly python2 invocations. All tests pass
on an Arch Linux system as of this commit.
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.
