batches: Don't let stderr clutter git output
Created by: eseliger
We parse lots of the things that git outputs, so having stderr randonly interfere with the output breaks that parsing. Since we can still get stderr from the exiterror, I don't think this is a regression at all in debuggability. This was found on k8s where parsing the patch would fail on a particular repo. Turns out git logged a warning.
Test plan
Verified the patch can now be parsed.