git-gui: Verify Tcl/Tk is new enough for our needs

git-gui: Verify Tcl/Tk is new enough for our needs

For quite a while we have been assuming the user is running on
a Tcl/Tk 8.4 or later platform.  This may not be the case on
some very old systems.  Unfortunately I am pretty far down the
path of using the Tcl/Tk 8.4 commands and options and cannot
easily work around them to support earlier versions of Tcl/Tk.
So we'll check that we are using the correct version up front,
and if not we'll stop with a related error message.

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