ReceiveCommits: Prevent NPE setting updated change list for edit commit

When pushing an amended commit with the %edit option, the ReplaceRequest
has null info, which causes NPE when trying to use it to get the subject
of the new patch set. This results in the message:

  "internal error while processing changes"

to be shown to the user instead of the "Updated Changes" list.

Use the ReplaceRequest's newCommit instead.

Also update AbstractPushForReview#testPushForMasterAsEdit to confirm that
the expected "Updated Changes" output is present.

Bug: Issue 4236
Change-Id: I82983219a3220d21f043fdc662f561e291fc0746

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

@@ -176,6 +176,10 @@ public abstract class AbstractPushForReview extends AbstractDaemonTest {
     r.assertOkStatus();
     edit = getEdit(r.getChangeId());
     assertThat(edit).isNotNull();
+    r.assertMessage("Updated Changes:\n  "
+        + canonicalWebUrl.get()
+        + r.getChange().getId()
+        + " " + edit.commit.subject + " [EDIT]\n");
   }
 
   @Test

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

@@ -722,7 +722,7 @@ public class ReceiveCommits {
       boolean edit = magicBranch != null && magicBranch.edit;
       for (ReplaceRequest u : updated) {
         addMessage(formatChangeUrl(canonicalWebUrl, u.change,
-            u.info.getSubject(), edit));
+            u.newCommit.getShortMessage(), edit));
       }
       addMessage("");
     }