When a commit has many tags, the tag icons in the graph display can
easily become so wide as to push the commit message off the right-hand
edge of the graph display pane. This changes the display so that if
there are more than 3 tags or they would take up more than a quarter
of the width of the pane, we instead display a single tag icon with
a legend inside it like "4 tags...". If the user clicks on the tag
icon, gitk then displays all the tags in the diff display pane.