Disable 'edit' for commit messages on merged changes

Adds a param to the function that computes the visibility of the edit
button for the commit message to allow for disabling when the change
is merged.

Bug: Issue 4353
Change-Id: I8a64f3c15ddd19c59dc4721470cd1bafbb36c152

polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js

diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
index b6c18e5..bc9cb50 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.js
@@ -66,7 +66,7 @@
       _hideEditCommitMessage: {
         type: Boolean,
         computed: '_computeHideEditCommitMessage(_loggedIn, ' +
-            '_editingCommitMessage)',
+            '_editingCommitMessage, _change)',
       },
       _latestCommitMessage: {
         type: String,
@@ -152,8 +152,10 @@
           }.bind(this));
     },
 
-    _computeHideEditCommitMessage: function(loggedIn, editing) {
-      if (!loggedIn || editing) { return true; }
+    _computeHideEditCommitMessage: function(loggedIn, editing, change) {
+      if (!loggedIn || editing || change.status === this.ChangeStatus.MERGED) {
+        return true;
+      }
 
       return false;
     },