git-gui: Add Explore Working Copy to the Repository menu

git-gui: Add Explore Working Copy to the Repository menu

Especially when cloning is finished, the Git GUI window pops up,
but there is not really much one can do within it - there needs to be
a way to easily start exploring and working with the new working copy
using the standard system interface: explorer.exe on Windows, open on
MacOS/X and xdg-open as a fallback (all modern Linux desktops).

This might be also a post-clone option instead (possibly opening the window
automagically) but I believe that this might be useful also in other
situations, e.g. you don't have to keep the working copy window around
if you work in multiple repositories.

This operation will not make sense on bare repositories.

Signed-off-by: Petr Baudis <petr.baudis@novartis.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>