Disable submit button if merge will fail

This feature aims to test the mergeability of each change tagging it
according to the result (true or false).
When the change page is load, the test is triggered and the sha1 of
the dest branch (tested against) is stored within the change table.
The submit button will be enabled or disabled according to the result.
The test is triggered only if it’s outdated.
As the branch evolves the sha1 stored at the moment of the test
becomes different from the last one merged into the branch, meaning
the test is outdated.

Bug: issue 381
Change-Id: I8bdb26db6fc35067b8c14b987984cfd80745f3d3

gerrit-common/src/main/java/com/google/gerrit/common/data/GerritConfig.java

@@ -41,6 +41,7 @@ public class GerritConfig implements Cloneable {
   protected Set<Account.FieldName> editableAccountFields;
   protected List<RegexFindReplace> commentLinks;
   protected boolean documentationAvailable;
+  protected boolean testChangeMerge;
 
   public String getRegisterUrl() {
     return registerUrl;
@@ -160,4 +161,12 @@ public class GerritConfig implements Cloneable {
   public void setDocumentationAvailable(final boolean available) {
     documentationAvailable = available;
   }
+
+  public boolean testChangeMerge() {
+    return testChangeMerge;
+  }
+
+  public void setTestChangeMerge(final boolean test) {
+    testChangeMerge = test;
+  }
 }