Automatically set patch description for message edits

This is a port-forward of https://gerrit-review.googlesource.com/c/93712,
but specific to the new PUT /changes/<id>/message API.

Bug: Issue 6502
Change-Id: I3ee5a609065d66d2d328670571355f9e4a329bb6

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/change/ChangeIT.java

diff --git a/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/change/ChangeIT.java b/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/change/ChangeIT.java
index 8607c02..275b06a 100644
--- a/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/change/ChangeIT.java
+++ b/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/change/ChangeIT.java
@@ -2926,6 +2926,7 @@
       RevisionApi rApi = gApi.changes().id(r.getChangeId()).current();
       assertThat(rApi.files().keySet()).containsExactly("/COMMIT_MSG", "a.txt");
       assertThat(getCommitMessage(r.getChangeId())).isEqualTo(newMessage);
+      assertThat(rApi.description()).isEqualTo("Edit commit message");
     }
   }
 

gerrit-server/src/main/java/com/google/gerrit/server/change/PutMessage.java

diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/PutMessage.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/PutMessage.java
index dcb5766..b0f571b 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/PutMessage.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/PutMessage.java
@@ -150,6 +150,7 @@
             psInserterFactory.create(resource.getControl(), psId, newCommit);
         inserter.setMessage(
             String.format("Patch Set %s: Commit message was updated.", psId.getId()));
+        inserter.setDescription("Edit commit message");
         inserter.setNotify(input.notify);
         inserter.setAccountsToNotify(notifyUtil.resolveAccounts(input.notifyDetails));
         bu.addOp(resource.getChange().getId(), inserter);