squash! Win32: support long paths

Squash in this commit text:
Test the case is when the full pathname length of a dir is close
to 260 (MAX_PATH).
Bug report and an original reproducer by Andrey Rogozhnikov:
https://github.com/msysgit/git/pull/122#issuecomment-43604199

Original-test-by: Andrey Rogozhnikov <rogozhnikov.andrey@gmail.com>
Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>