Use Patch.COMMIT_MSG instead of redefining this constant in the plugin

Change-Id: Ia381bb68c2b152ce9c211c17d7ea080eff2ab484
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

src/main/java/com/googlesource/gerrit/plugins/xdocs/client/PatchSetSelectBox.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/xdocs/client/PatchSetSelectBox.java b/src/main/java/com/googlesource/gerrit/plugins/xdocs/client/PatchSetSelectBox.java
index 21190a2..72ba99b 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/xdocs/client/PatchSetSelectBox.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/xdocs/client/PatchSetSelectBox.java
@@ -15,6 +15,7 @@
 package com.googlesource.gerrit.plugins.xdocs.client;
 
 import com.google.gerrit.plugin.client.Plugin;
+import com.google.gerrit.reviewdb.client.Patch;
 import com.google.gwt.core.client.GWT;
 import com.google.gwt.core.client.JsArray;
 import com.google.gwt.http.client.URL;
@@ -37,8 +38,6 @@
     UNIFIED_DIFF
   }
 
-  private static final String COMMIT_MSG = "/COMMIT_MSG";
-
   private final DiffView diffView;
   private final DisplaySide side;
   private final ChangeInfo change;
@@ -79,7 +78,7 @@
       add(createLink(list.get(i)));
     }
 
-    if (!COMMIT_MSG.equals(path)) {
+    if (!Patch.COMMIT_MSG.equals(path)) {
       add(createDownloadLink());
     }