gitk: Fix linehtag undefined error with file highlighting

Occasionally gitk will throw a Tcl error complaining that linehtag(n) is undefined when. It happens when the commit list is still growing (e.g. when updating the commit list) and gitk is set to highlight commits that affect certain file(s). What happens is that the changes to the commit list set need_redisplay to indicate that the display needs to be redrawn. That causes the next call to drawcommits to call clear_display, which unsets iddrawn and thus ensures that readfhighlight won't call bolden on any rows that have moved. However, it is possible for readfhighlight to be called after the commit list has changed but before drawcommits has run, meaning that readfhighlight will potentially think that rows have been drawn when they haven't, because of the change in the id -> row mapping (and the fact that iddrawn is indexed by id but line[hnd]tag are indexed by row number). This fixes it (and also optimizes things a little) by making bolden and bolden_name check need_redisplay before doing anything. If need_redisplay is set, then there is no point doing anything because the whole display is about to get cleared and redrawn, and it avoids looking up line[hn]tag using stale row numbers. Signed-off-by: Paul Mackerras <paulus@samba.org>