CreateBranch: Test creation of a conflicting branch

If a branch refs/heads/test exists it's not possible to create another
branch under that namespace (e.g. refs/heads/test/foo).

Add a test for this to increase the test coverage of the CreateBranch
REST endpoint.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: Id6d004e802775bf1d8ebb34860ebe5554157a2bc

javatests/com/google/gerrit/acceptance/rest/project/CreateBranchIT.java

@@ -140,6 +140,20 @@ public class CreateBranchIT extends AbstractDaemonTest {
     }
   }
 
+  @Test
+  public void conflictingBranchAlreadyExists_Conflict() throws Exception {
+    assertCreateSucceeds(testBranch);
+    BranchNameKey testBranch2 = BranchNameKey.create(project, testBranch.branch() + "/foo/bar");
+    assertCreateFails(
+        testBranch2,
+        ResourceConflictException.class,
+        "Cannot create branch \""
+            + testBranch2.branch()
+            + "\" since it conflicts with branch \""
+            + testBranch.branch()
+            + "\"");
+  }
+
   @Test
   public void createBranchByProjectOwner() throws Exception {
     grantOwner();