Would like to reopen this ticket.

Hello,

This forum thread is not closed and you can still write there.

Do you still experience problems with running PhpStorm as a Diff tool?

Yeah:  

If I attempt:   /usr/local/bin/pstorm diff README.acquia .gitignore

I get:  No IDE instance has been found. New one will be started.

diff never opens.

You can close. I got this working fine for me by adding an alias for:  /usr/local/bin/pstorm diff