document: tell whether a document holds a change without building it

With the policy's reads kept, judging a change still costs about one
read (some 3.5 µs), and so does judging without a policy: what is left
is At::new, which asks get_change_by_hash for each head. Automerge
builds the change again from the document's operations to answer, at
a cost growing with its history. Screening asks it again for every
candidate and every dependency, and sync for heads.

Ask by the missing dependencies instead (changes::holds), which look
the hash up: one the document holds is not among them. A change queued
in the document, waiting for its dependencies, isn't either; but then
some are missing, and only then is it asked the slow way. Every check
of whether a document holds a change goes through it.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/remake/snapshot.rs b/src/remake/snapshot.rs
index ee0a2cd..dc61810 100644
--- a/src/remake/snapshot.rs
+++ b/src/remake/snapshot.rs
@@ -274,7 +274,7 @@ pub(crate) fn build(
 ) -> Result<Snapshot, SnapshotError> {
     // fork_at with unknown heads would quietly build something else.
     for head in params.base_heads {
-        if source.get_change_by_hash(head).is_none() {
+        if !crate::document::changes::holds(source, head) {
             return Err(SnapshotError::MissingBase(*head));
         }
     }