InMemoryRepositoryManager: Increment repository use count

When the same repository is opened more than once its use count
should be incremented. If it is not, closing it more than once
will result in the warning from JGit:

  close() called when useCnt is already zero

Change-Id: I08ed7b544bef942a595f2a85030e6d4faa90f3f2
diff --git a/gerrit-server/src/test/java/com/google/gerrit/testutil/InMemoryRepositoryManager.java b/gerrit-server/src/test/java/com/google/gerrit/testutil/InMemoryRepositoryManager.java
index f98d63f..e7bd8f8 100644
--- a/gerrit-server/src/test/java/com/google/gerrit/testutil/InMemoryRepositoryManager.java
+++ b/gerrit-server/src/test/java/com/google/gerrit/testutil/InMemoryRepositoryManager.java
@@ -121,6 +121,7 @@
       throws RepositoryNotFoundException {
     Repo repo = repos.get(normalize(name));
     if (repo != null) {
+      repo.incrementOpen();
       return repo;
     }
     throw new RepositoryNotFoundException(name.get());