virtualenv #904: Fix for users of fish shell

[Ian Boarder]

Hello. I have submitted a patch that only touches the activate.fish file and provides a fix for fish users who may have subprocesses that use virtualenv.

It's a relatively small use case, but the fix is also very isolated.

Does anyone have any feedback for me, or can this be merged? Thank you!

https://github.com/pypa/virtualenv/pull/904