git-gui: Include the Tcl/Tk version in the about dialog.

Users may need to know what version of Tcl they are running git-gui
under, in case there is an interesting interface quirk or other
compatability problem we don't know about right now that we may
need to explore (and maybe fix).  Since its simple enough to show
a line with this version data we should do so.

We also try to reduce the amount of text shown as often the Tcl and Tk
version numbers will be identical; when this happens we should only show
the one version number.

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