cs-bisection-step: Mark which revision(s) changed in each node

cs-bisection-step: Mark which revision(s) changed in each node

We compare the revision rtuple elements with each of the parents, and
mark changes by surrounding the revision id with *asterisks*.

In the SVG output, we have to strip these when generating the URL, and
we can show this by emboldening the relevant revision instead.  This
involves changing the ther output to be of normal weight by deleting
the emboldening output by dot.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>