Change API calls to change.currentRevision instead use raw string.

This is so that if change.currentRevision is null, we don't get an NPE.

Change-Id: I37d517b35355fda8e237ea549b2ad9b33eddd3f5
diff --git a/src/main/java/com/googlesource/gerrit/plugins/automerger/DownstreamCreator.java b/src/main/java/com/googlesource/gerrit/plugins/automerger/DownstreamCreator.java
index f56d464..6854143 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/automerger/DownstreamCreator.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/automerger/DownstreamCreator.java
@@ -79,6 +79,7 @@
   private static final String MERGE_CONFLICT_TAG = "autogenerated:MergeConflict";
   private static final String SUBJECT_PREFIX = "automerger";
   private static final String SKIPPED_PREFIX = "skipped";
+  private static final String CURRENT = "current";
 
   protected GerritApi gApi;
   protected ConfigLoader config;
@@ -155,7 +156,7 @@
               "Automerger prevented the topic from changing. Topic can only be modified on "
                   + "non-automerger-created CLs to a non-empty value.");
           reviewInput.notify = NotifyHandling.NONE;
-          gApi.changes().id(change._number).revision(change.currentRevision).review(reviewInput);
+          gApi.changes().id(change._number).revision(CURRENT).review(reviewInput);
         } catch (RestApiException e) {
           log.error("Failed to prevent setting empty topic for automerger plugin.", e);
         }
@@ -336,13 +337,13 @@
       try {
         gApi.changes()
             .id(mdsMergeInput.changeNumber)
-            .revision(mdsMergeInput.currentRevision)
+            .revision(CURRENT)
             .review(reviewInput);
       } catch (AuthException e) {
         reviewInput.labels = null;
         gApi.changes()
             .id(mdsMergeInput.changeNumber)
-            .revision(mdsMergeInput.currentRevision)
+            .revision(CURRENT)
             .review(reviewInput);
       }
     }
@@ -654,7 +655,7 @@
     reviewInput.notify = NotifyHandling.NONE;
     reviewInput.tag = AUTOMERGER_TAG;
     try {
-      gApi.changes().id(change.id).revision(change.currentRevision).review(reviewInput);
+      gApi.changes().id(change.id).revision(CURRENT).review(reviewInput);
     } catch (AuthException e) {
       log.error("Automerger could not set label, but still continuing.", e);
     }