git-gui: fix "Explore Working Copy" for Windows again

It has already been fixed in commit 454efb47 (git-gui (Win): make "Explore Working Copy" more robust, 2009-04-01), but has been broken in commit 21985a11 (git-gui: handle non-standard worktree locations, 2010-01-23) by accidentally replacing too much with a new variable. The problem can be reproduced when starting git-gui from within a subdirectory. The solution is to convert the path name, explorer.exe is invoked with, to a platform native name. Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de> Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>