Improve click behavior on links in branch log

Rather than having the link's parent table cell's onclick handler
completely override the link's behavior, make it simply click the link
(unless the event handler is invoked for an event propagated from the
link.)  This means that direct clicks on the link are unaffected and thus
that all special behaviors of the browser (middle-click, shift-click,
et c.) work normally.  In addition, in some browsers, some such behavior
(e.g. shift-click in Presto) will work on the table cell as well.

Also make sure that commit selection is always reset on mouseup.