gitk: Fix bug introduced by "gitk: Fix "wrong # coordinates" error on reload"

Commit 94503a66c5 ("gitk: Fix "wrong #
coordinates" error on reload") was correct as far as it went, but
introduced a problem because it didn't also clear out boldrows and
boldnamerows in clear_display.  This resulted in Tcl errors after
scrolling through the graph for a while if some rows were highlighted.
This fixes it.

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

gitk

@@ -5125,7 +5125,7 @@ proc drawvisible {} {
 proc clear_display {} {
     global iddrawn linesegs need_redisplay nrows_drawn
     global vhighlights fhighlights nhighlights rhighlights
-    global linehtag linentag linedtag
+    global linehtag linentag linedtag boldrows boldnamerows
 
     allcanvs delete all
     catch {unset iddrawn}
@@ -5133,6 +5133,8 @@ proc clear_display {} {
     catch {unset linehtag}
     catch {unset linentag}
     catch {unset linedtag}
+    set boldrows {}
+    set boldnamerows {}
     catch {unset vhighlights}
     catch {unset fhighlights}
     catch {unset nhighlights}