We look for a Git for Windows installation to use its git config,
so that clients built on libgit2 can interoperate with the Git for
Windows CLI (and clients that are built on top of _it_).

Look for `git` both in the `PATH` and in the registry.  Use the _first_
git install in the path, and the first git install in the registry.

Look in both the `etc` dir and the architecture-specific `etc` dirs
(`mingw64/etc` and `mingw32/etc`) beneath the installation root.

Prefer the git in the `PATH` to the git location in the registry so that
users can override that.

Include more tests for this behavior.