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

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>