SubmoduleOp: Fix formatting of submodule update commit message

When multiple submodules are updated, the commit message does not
include newlines between the list of updated submodules.

Add newlines, and update the test to include an assertion on the
commit message.

Bug: Issue 4889
Change-Id: I0756a87034a0b2b432a8339596147d765aa8cd4d

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/git/SubmoduleSubscriptionsWholeTopicMergeIT.java

@@ -308,6 +308,52 @@ public class SubmoduleSubscriptionsWholeTopicMergeIT extends AbstractSubmoduleSu
     expectToHaveSubmoduleState(superRepo, "master", "sub2", sub2, "master");
     expectToHaveSubmoduleState(superRepo, "master", "sub3", sub3, "master");
 
+    String sub1HEAD =
+        sub1.git()
+            .fetch()
+            .setRemote("origin")
+            .call()
+            .getAdvertisedRef("refs/heads/master")
+            .getObjectId()
+            .name();
+
+    String sub2HEAD =
+        sub2.git()
+            .fetch()
+            .setRemote("origin")
+            .call()
+            .getAdvertisedRef("refs/heads/master")
+            .getObjectId()
+            .name();
+
+    String sub3HEAD =
+        sub3.git()
+            .fetch()
+            .setRemote("origin")
+            .call()
+            .getAdvertisedRef("refs/heads/master")
+            .getObjectId()
+            .name();
+
+    if (getSubmitType() == SubmitType.MERGE_IF_NECESSARY) {
+      expectToHaveCommitMessage(
+          superRepo,
+          "master",
+          "Update git submodules\n\n"
+              + "* Update "
+              + name("sub3")
+              + " from branch 'master'\n  to "
+              + sub3HEAD
+              + "\n\n* Update "
+              + name("sub2")
+              + " from branch 'master'\n  to "
+              + sub2HEAD
+              + "\n\n* Update "
+              + name("sub1")
+              + " from branch 'master'\n  to "
+              + sub1HEAD);
+    }
+
     superRepo
         .git()
         .fetch()

gerrit-server/src/main/java/com/google/gerrit/server/git/SubmoduleOp.java

@@ -375,8 +375,13 @@ public class SubmoduleOp {
     PersonIdent author = null;
     DirCache dc = readTree(or.rw, currentCommit);
     DirCacheEditor ed = dc.editor();
+    int count = 0;
     for (SubmoduleSubscription s : targets.get(subscriber)) {
+      if (count > 0) {
+        msgbuf.append("\n\n");
+      }
       RevCommit newCommit = updateSubmodule(dc, ed, msgbuf, s);
+      count++;
       if (newCommit != null) {
         if (author == null) {
           author = newCommit.getAuthorIdent();