The basic problem is that sometimes when I do
git pull upstream master
I get
remote: Counting objects: 172, done.
remote: Comp
I get that some time has passed, but I found this searching for my own very similar issue, using the cygwin platform built into the MobaXterm app. Was pulling my nonexistent hair out trying to figure it out, in the end it was fixed by a simple restart.
I double verified that this worked with a second Windows 10 machine, a new installation of MobaXterm + git apps, and an attempt to pull from a github repo. Same error: "error: cannot fork() for index-pack: Resource temporarily unavailable" was achieved, and a simple computer reboot fixed it.