datastore: don't nil out transaction ID on retry
datastore: don't nil out transaction ID on retry

If we're going to retry a transaction because it aborted, we
want its ID so it won't lose its priority. But Commit was setting
the ID to nil unconditionally.

Change Commit to leave the ID intact for retry.

Fixes #1137.

Change-Id: Ifb62bafc0d230da798d332ebd05150571dc14641
Reviewed-on: https://code-review.googlesource.com/32770
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go