eclipse-iplog: Use contribution rather than bug element

Wayne changed the schema to no longer be dependent upon the Bugzilla
notion of a contribution, but instead be more generic and better
support systems like Gerrit Code Review.  Update our output to
use the <contribution> element and include a link to the change
in Gerrit.

Change-Id: Ibc8a436918bd8e7597dc17743824201a74bce09b
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>