less-scary log message when testing if a commit is present

[ktdreyer]

Here is an example where a ref was not yet fetched locally:

$ downstream-cherry-picker https://github.com/ceph/ceph/pull/17101 1483224
INFO: + git cat-file -e 9de0f8f68aab7e90b19e97570fbc0075204231d8^{commit}
fatal: Not a valid object name 9de0f8f68aab7e90b19e97570fbc0075204231d8^{commit}
INFO: First upstream commit not found. Fetching from github.com.
...

This "fatal:" message is too scary-sounding. We should filter that particular message out.