mingw: factor out code to set stat() data

mingw: factor out code to set stat() data

In our fstat() emulation, we convert the file metadata from Win32 data
structures to an emulated POSIX structure.

To structure the code better, let's factor that part out into its own
function.

Note: it would be tempting to try to unify this code with the part of
do_lstat() that does the same thing, but they operate on different data
structures: BY_HANDLE_FILE_INFORMATION vs WIN32_FILE_ATTRIBUTE_DATA. So
unfortunately, they cannot be unified.

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