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>

git-gui.sh

@@ -20,6 +20,22 @@ You should have received a copy of the GNU General Public License
 along with this program; if not, write to the Free Software
 Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA}
 
+######################################################################
+##
+## Tcl/Tk sanity check
+
+if {[catch {package require Tcl 8.4} err]
+ || [catch {package require Tk  8.4} err]
+} {
+	catch {wm withdraw .}
+	tk_messageBox \
+		-icon error \
+		-type ok \
+		-title "git-gui: fatal error" \
+		-message $err
+	exit 1
+}
+
 ######################################################################
 ##
 ## configure our library