git-gui (Win): make "Explore Working Copy" more robust

git-gui (Win): make "Explore Working Copy" more robust

Starting the Explorer from the git-gui menu "Explore Working Copy"
didn't work, when git-gui was started via Windows Explorer shell
extension (Git GUI Here) from a directory within the project.
The Explorer raised an error message like this:

    Path "C:/somedir/worktree" is not available or not a directory

It worked when started from the project directory itself, because then
the path argument for the Explorer was just '.' (current directory)
without any problematic forward slashes.

To make it work, convert the path given as argument to explorer.exe to
its native format with backslashes.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>