Support topic branch tags on changes

The topic tag can be attached to a change during upload by suffixing
the 'refs/for/branch_name' with the topic label, treating the branch
as though it were a directory.

For example, to tag 'exp/nosql' onto a change headed for the
'sandbox/future-stuff' branch, git push can be used as:

  git push URL HEAD:refs/for/sandbox/future-stuff/exp-nosql

If the topic is supplied, it is displayed in the branch column of
a change, in parens.  If no topic was set, only the branch is shown.

Bug: issue 51
Change-Id: I07d6c137fc9aefa8c1ee1652bf1e7bcde9d33674
Signed-off-by: Shawn O. Pearce <sop@google.com>

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

@@ -27,6 +27,7 @@ public class ChangeInfo {
   protected Change.Status status;
   protected ProjectInfo project;
   protected String branch;
+  protected String topic;
   protected boolean starred;
   protected Timestamp lastUpdatedOn;
   protected String sortKey;
@@ -42,6 +43,7 @@ public class ChangeInfo {
     status = c.getStatus();
     project = new ProjectInfo(c.getProject());
     branch = c.getDest().getShortName();
+    topic = c.getTopic();
     lastUpdatedOn = c.getLastUpdatedOn();
     sortKey = c.getSortKey();
   }
@@ -74,6 +76,10 @@ public class ChangeInfo {
     return branch;
   }
 
+  public String getTopic() {
+    return topic;
+  }
+
   public boolean isStarred() {
     return starred;
   }