Try harder to find global config file

Do not stop looking for the user's global .gitconfig file when the path exists, but there is no .gitconfig file.

As discussed, this is a deviation from msys git behavior, but the proposed change seems to reflect the expected behavior from libgit2. The current behavior has already caused issues (e.g. #952). Instead of consumers working around the current behavior, this updates libgit2 to try harder to find the user's global config file on Windows.

/cc @arrbee, @Haacked, @clboles, @phkelley, @ethomson