PushCertificateStore: Don't add no-op command to batch

PushCertificateStore: Don't add no-op command to batch

If no refs match the input list and we are writing to a batch,
the returned new commit from write() will match the current commit.
Adding a command to the batch for this case is harmless as it will
succeed, but it's more straightforward to just skip adding a command
in this case.

Add tests or the combination of saving matching refs and saving to a
batch.

Change-Id: I6837389b08e6c80bc2d4c9e9c506d07293ea5fb2