gitk: Fix "wrong # coordinates" error on reload

This fixes the Tk error "wrong # coordinates: expected 0 or 4, got 2"
that sometimes occurred when reloading.  The problem was that we didn't
unset the variables containing the canvas item id numbers for the
displayed rows when we cleared the canvases.  Thus make_secsel would
think it had something to do when it didn't.

Thanks to Michele Ballabio for finding a way to trigger the bug
reliably.

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

gitk

@@ -5125,10 +5125,14 @@ proc drawvisible {} {
 proc clear_display {} {
     global iddrawn linesegs need_redisplay nrows_drawn
     global vhighlights fhighlights nhighlights rhighlights
+    global linehtag linentag linedtag
 
     allcanvs delete all
     catch {unset iddrawn}
     catch {unset linesegs}
+    catch {unset linehtag}
+    catch {unset linentag}
+    catch {unset linedtag}
     catch {unset vhighlights}
     catch {unset fhighlights}
     catch {unset nhighlights}