review-classifier: check that a commit message's claimed changes are in the diff

message-claims-match-diff is the reverse of one-purpose: a README, doc
page, test or rename the message names must be in the diff. The rules
file then holds the ten-rule cap, so the new-rule merge test appends to
the file minus its last entry and no longer depends on how many rules the
file holds.

.hyperhive/review-classifier.yml

@@ -82,3 +82,12 @@ classifications:
       does, and nothing unrelated is bundled in.
     polarity: positive
     threshold: 65
+  - name: message-claims-match-diff
+    description: >-
+      Everything a commit message says it changes in a file is in the diff:
+      a doc page, a README, a test, a rename or a removal that it names.
+      This is the reverse of one-purpose, which checks that the diff has
+      nothing the messages do not mention. A commit message that names no
+      such change makes the question not applicable.
+    polarity: positive
+    threshold: 65

swarm-controller/src/review_classifier.rs

@@ -1336,6 +1336,7 @@ classifications:
                 "tests-with-behaviour",
                 "docs-updated",
                 "one-purpose",
+                "message-claims-match-diff",
             ]
         );
     }

swarm-controller/src/review_classifier/open_pr.rs

@@ -537,22 +537,27 @@ mod tests {
     #[test]
     fn a_new_rule_leaves_every_existing_entry_as_it_was_written() {
         let [yml, _] = RULES_PATHS;
-        let before = parse_rules(yml, OWN.as_bytes()).expect("this repo's rules parse");
+        // This repo's file without its last entry: the file may already hold
+        // the cap, and the property under test is how a new entry is appended,
+        // not how many entries the file has today.
+        let last = OWN.rfind("\n  - name:").expect("a last entry") + 1;
+        let own = &OWN[..last];
+        let before = parse_rules(yml, own.as_bytes()).expect("this repo's rules parse");
         let new = rule(
             "no-todo",
             "The diff adds a TODO or FIXME comment without a tracker reference next to it, \
              which leaves the follow-up with nobody to own it.",
         );
-        let merged = merge(Some(&file(yml, OWN)), vec![new.clone()]).expect("merges");
+        let merged = merge(Some(&file(yml, own)), vec![new.clone()]).expect("merges");
         let text = String::from_utf8(merged.content).expect("utf-8");
-        assert!(text.starts_with(OWN), "{text}");
+        assert!(text.starts_with(own), "{text}");
         assert_eq!(merged.sha.as_deref(), Some("b1"));
         assert_eq!(merged.message, "review classifier: add no-todo");
         let after = parse_rules(yml, text.as_bytes()).expect("parses");
         assert_eq!(after, [before, vec![new.clone()]].concat());
         assert!(text.lines().all(|l| l.len() <= YAML_WIDTH), "{text}");
 
-        let unterminated = OWN.trim_end();
+        let unterminated = own.trim_end();
         let merged = merge(Some(&file(yml, unterminated)), vec![new]).expect("merges");
         let text = String::from_utf8(merged.content).expect("utf-8");
         assert!(