Silence noisy ProtectedBranches integration test output

We want our test runs to be free of noise, and this test was outputting:

    fatal: ambiguous argument '': unknown revision or path not in the
    working tree.
    Use '--' to separate paths from revisions, like this:
    'git <command> [<revision>...] -- [<file>...]'

Fix with simple redirection.

Author: sds

spec/integration/protected_branches_spec.rb

@@ -53,7 +53,7 @@
   end
 
   shared_context 'local branch up-to-date' do
-    before { `git rebase --keep-empty origin/#{remote_ref}` }
+    before { `git rebase --keep-empty origin/#{remote_ref} > #{File::NULL} 2>&1` }
   end
 
   shared_context 'ProtectedBranches enabled' do