Extreme performance regression in 1.4.x

[ofek]

Currently we are pinning to 1.3.0 because starting in version 1.4.1 the build time increased by 70% (1.4.0 specifically was worse for some reason). I tried disabling the new parallel processing option just in case but that increases the time substantially.

I think the issue fundamentally is the use of multiprocessing. As far as I can tell at a quick glance basically everything in memory is required to run that utility function and therefore every process has a large copy of the current process. I think a better way for that function specifically would be to run multiple Popen instances of git and wait for them all to complete.