A dependent rebuilt because its base moved, or a module that packages another repository's source,
keeps its source commit; the catalogue announces no move for it, and its machines kept the old image
until somebody pushed — forty-three modules after every runtime-image merge (hq issue 189). When a tier
is built, the plan now sends the machines of every module in it whose policy rolls out, once, moved
commit or not; a module whose policy records is built and left, as its policy says. The gate between
tiers waits as before for the ones a later tier is built by.
jschoubben