solver can spawn multiple processes per HTTP request, but
"git apply" failures are needlessly noisy due to corrupt
patches. We also don't want to silence "git ls-files"
or "git update-index" errors using $env->{'qspawn.quiet'},
either, so this granularity is needed.
Admins can check for 500 errors in access logs to detect
(and reproduce) solver failures, anyways, so there's no
need to log every time "git apply" rejects a corrupt patch.