git-gui: Keep the UI responsive while counting objects in clone

git-gui: Keep the UI responsive while counting objects in clone

If we are doing a "standard" clone by way of hardlinking the
objects (or copying them if hardlinks are not available) the
UI can freeze up for a good few seconds while Tcl scans all
of the object directories.  This is espeically noticed on a
Windows system when you are working off network shares and
need to wait for both the NT overheads and the network.

We now show a progress bar as we count the objects and build
our list of things to copy.  This keeps the user amused and
also makes sure we run the Tk event loop often enough that
the window can still be dragged around the desktop.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>