This is where portable git stores the global configuration which we can
use to adhere to it even though git isn't quite installed on the system.

Don't coalesce all errors into ENOENT.  At least identify EACCES.
All callers should be handling this case already, as the POSIX
`lstat` will return this.

win32: ensure hidden files can be staged

With Visual Studio versions 2008 and older they ignore the full path to files and only check
the basename of the file to find a collision. Additionally, having duplicate basenames can break
other build tools like GYP.

This fixes https://github.com/libgit2/libgit2/issues/3356

git__getenv: utf-8 aware env reader

Introduce `git__getenv` which is a UTF-8 aware `getenv` everywhere.
Make `cl_getenv` use this to keep consistent memory handling around
return values (free everywhere, as opposed to only some platforms).