When we renamed HgEclipse to MercurialEclipse there was a need for a transition plugin which was basically an empty HgEclipse 1.5.1 version. This was used to inform the customers about this change, and was effectively uninstalling HgEclipse 1.5.0 and previous versions.
This plugins seems now obsolete, probably everybody has switched to MercurialEclipse since. So let's drop these plugins from the update sites and dependencies. So delete the "hgeclipse-transition-plugin" and the associated "hgeclipse-transition-feature".