mingw: handle a file owned by the Administrators group correctly

mingw: handle a file owned by the Administrators group correctly

When an Administrator creates a file or directory, the created
file/directory is owned not by the Administrator SID, but by the
_Administrators Group_ SID. The reason is that users with administrator
privileges usually run in unprivileged ("non-elevated") mode, and their
user SID does not change when running in elevated mode.

This is is relevant e.g. when running a GitHub workflow on a build
agent, which runs in elevated mode: cloning a Git repository in a script
step will cause the worktree to be owned by the Administrators Group
SID, for example.

Let's handle this case as following: if the current user is an
administrator, Git should consider a worktree owned by the
Administrators Group as if it were owned by said user.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>