msvc: opendir: do not start the search

msvc: opendir: do not start the search

compat/mingw.c's readdir expects to be the one that starts the search,
and if it isn't, then the first entry will be missing or incorrect.

Fix this by removing the call to _findfirst, and initializing dd_handle
to INVALID_HANDLE_VALUE.

At the same time, make sure we use FindClose instead of _findclose,
which is symmetric to readdir's FindFirstFile. Take into account that
the find-handle might already be closed by readdir.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>