This is a pragmatic temporary solution to allow us to more quickly resolve
an upstream regression in go-getter locally within Terraform, so that the
work to upstream it for other callers can happen asynchronously and with
less time pressure.

This commit doesn't yet include any changes to address the bug, and
instead aims to be functionally equivalent to getter.GitGetter. A
subsequent commit will then address the regression, so that the diff of
that commit will be easier to apply later to the upstream to get the same
effect there.