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.

swarm-controller/src/review_classifier.rs

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