check-manifest: Very slow git submodule checking in a Jenkins environment on Windows

When I added git submodule checking, my Jenkins job run time went from 10 minutes up to 1 hour and 45 minutes. Ouch.

It turns out that every invocation of git submodule ... takes 50 seconds – but only inside Jenkins. I cannot reproduce this from a command shell on the same machine.

I suspect a different %PATH% setting might be the cause. Perhaps Windows systems take a long time not finding executables on a long %PATH%?

(Meanwhile, I implemented a workaround, but that still leaves me a job that takes 30 minutes instead of the old 10 minutes.)