Commands relying on $VIM env var stop working properly when using dispatch

[foca]

Specifically, the minitest-reporters gem relies on $VIM being set to identify when running through vim to disable any fancy formatting and just outputting test results in a format that can be parsed back into the quicklist.

Since dispatch#isolate removes VIM, this then breaks, leading to garbled output. The quicklist then works, but looks ugly with color codes printed in between the output:

Could dispatch be made configurable to add a list of variables that can be kept while isolating? (or can it just be made available to the underlying process instead of stripping it from the env?)

[tpope]

The reason we strip it out is so you can :Start a different version of Vim with everything going to hell. There isn't a reason to do that with :Dispatch, so I think that's the condition to operate on. I would expect this to be a simple change to dispatch#isolate to add a conditional based on a:request.action.