gitk: Check that we are running on at least Tcl/Tk 8.4

gitk: Check that we are running on at least Tcl/Tk 8.4

This checks that we have Tcl/Tk 8.4 or later, and puts up an error
message in a window and quits if not.

This was prompted by a patch submitted by Steffen Prohaska, but is
done a bit differently (this uses package require rather than
looking at [info tclversion], and uses show_error to display the
error rather than printing it to stderr).

Signed-off-by: Paul Mackerras <paulus@samba.org>