mingw: implement `stat()` with symlink support

mingw: implement `stat()` with symlink support

With respect to symlinks, the current `mingw_stat()` implementation is
almost identical to `mingw_lstat()`: except for the file type (`st_mode
& S_IFMT`), it returns information about the link rather than the target.

Implement `mingw_stat()` by opening the file handle requesting minimal
permissions, and then calling `GetFileInformationByHandle()` on it. This
way, all links are resolved by the Windows file system layer.

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