git-gui: gui.autoexplore makes explorer to pop up automatically after picking

git-gui: gui.autoexplore makes explorer to pop up automatically after picking

Especially for Windows users used to work with the Windows Explorer,
it is very useful when after picking a repository (either opening
a local one or initializing/cloning a new one) in the "intro" window,
the explorer view of the working copy pops up along the standard
Git GUI window, so that the users can, well, actually work with
the repository.

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