support for executing an external Git environment

[shiftkey]

Fixes #68

@smashwilson I'm not sure if this is the whole solution as you saw it, but here's what it looks like in the test suite:

cross-env LOCAL_GIT_DIRECTORY=./git/ mocha --require ts-node/register test/fast/*.ts test/auth/*.ts

There's still some other assumptions around portable Git baked into this that might break for other usages, depending on how Git is setup on the user's machine:

• GIT_EXEC_PATH - this is set at compile time, and maybe we shouldn't set this if it's already defined like we currently are. It's hard to know whether this should be set at runtime without invoking git --exec-path
• GIT_TEMPLATE_DIR - this might fail if not set, I can't really remember exactly. It's related to initializing repositories.
• PREFIX - defines where Git is currently installed, to override lookups to the default location of /usr/local

Maybe we need some more control here and better respect for existing environment variables, as I'd love to keep the defaults just working with the embedded Git environment.

[smashwilson]

👍 Looks perfectly reasonable to me. My only comments are on code that was just refactored as part of this PR 😄