Show View Diff button for merge change messages

Upon submitting a change with unreviewed files(when approval is sticky),
Gerrit sends a change message containing the latest approved patchset.
Add a View Diff button to this change message which shows the diff
between this approved patchset and the latest patchset.

Change-Id: I8a86c8ebe54726100aa8ca97c2c11e29673a1df3
diff --git a/polygerrit-ui/app/constants/constants.ts b/polygerrit-ui/app/constants/constants.ts
index b0f87d7..03d5000 100644
--- a/polygerrit-ui/app/constants/constants.ts
+++ b/polygerrit-ui/app/constants/constants.ts
@@ -53,6 +53,7 @@
   TAG_SET_WIP = 'autogenerated:gerrit:setWorkInProgress',
   TAG_SET_ASSIGNEE = 'autogenerated:gerrit:setAssignee',
   TAG_UNSET_ASSIGNEE = 'autogenerated:gerrit:deleteAssignee',
+  TAG_MERGED = 'autogenerated:gerrit:merged',
 }
 
 /**