Detect xcode git shim and handle appropriately

[maxfierke]

We need to ignore the default CLT shim for git and find the real path ourselves to properly skip git fetches before git is available. Otherwise, it seems present, but is actually missing and will pop up the Xcode CLT installer dialog and fail to fetch git resources.

This is based broadly on what is being done by Homebrew here: https://github.com/Homebrew/install/blob/52237ea74f41d2456a5b1bd3e4c2b0cb94485c3c/install#L118-L151