Only override GIT_SSH_COMMAND if the user has not already set it

[dustinleblanc]

This could resolve #334

[rvtraveller]

I like this approach and am generally on board with the change. The one thing that I think would be useful is to print something out when the user has the GIT_SSH_COMMAND environment variable set. Something like "Using your custom GIT_SSH_COMMAND variable instead of the Build Tools default" or similar.

[greg-1-anderson]

I'd recommend that we instead get the user's GIT_SSH_COMMAND setting and then iterate through each of our custom entries:

• -o UserKnownHostsFile=/dev/null
• -o StrictHostKeyChecking=no
• -o LogLevel=QUIET

We could make a map with entries like 'UserKnownHostsFile' => '/dev/null', and then composer the -o KEY=VALUE line if KEY does not already appear in the user's GIT_SSH_COMMAND.

That would be safer, and more likely to produce the desired result.

If GIT_SSH_COMMAND is not defined, we could just start with ssh and then run the same algorithm to add all of the config items.