A header included by fourteen translation units was hashed once per dependent,
and every built action hashed its inputs twice: once to decide it was stale and
again to store the new key. Digests are now memoised for the run, and the
scheduler drops the entries for an action's outputs and depfile once it
finishes, so a freshly written file is never read from the memo.
Checks each node's key at the moment it becomes ready rather than up front,
because an input may have been produced by a node that ran seconds earlier.
This is what gives early cutoff.
milner