mingw: improve performance of mingw_unlink()

Update mingw_unlink() to first try to delete the file with existing permissions before trying to force it. Windows throws an error when trying to delete a read-only file. The mingw_unlink() compatibility wrapper always tries to _wchmod(666) the file before calling _wunlink() to avoid that error. However, since most files in the worktree are already writable, this is usually wasted effort. Update mingw_unlink() to just call DeleteFileW() directly and if that succeeds return. If that fails, fall back into the existing code path to update the permissions and use _wunlink() to get the existing error code mapping. Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>