How to fix "address space is already occupied" error on fetch/commit?

Rebasing my Cygwin install solved this for me. I followed the doc Rebaseall, running dash as admin from Windows Explorer and then issuing the following:

/usr/bin/rebaseall -v

P.S.: No Cygwin service should be running for this to work.

Take a look at this Cygwin FAQ section and this too. In my case, disabling Avast's Behavior and Filesystem Realtime shields and then restarting Cygwin was the answer.

So, probably, you are running windows programs that hooks their DLLs into forked processes, disrupting addresses and thus causing fork to malfunction.