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

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

Commit 94503a66c56c935e77a8fbe3622f1f56b7134ccc ("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>