Fix PlotCommit for commits with duplicate parents

Fix PlotCommit for commits with duplicate parents

JGit allows to create commits which have duplicate parents: e.g. a
commit X has first parent Y and second parent Y. Such commits are not
handled correctly by PlotCommit leading to wrong display of the history
in EGit. In such cases there is a never ending passing line drawn beside
all commits younger than the commit with duplicate parents. This commit
fixes this by explicitly checking for duplicate parents.

In a different commit we should fix JGit not to create commits with
duplicate parents. I think native git also doesn't allow such commits,
although history display in native git (gitk, git log --graph) is not
damaged by such commits.

Change-Id: Ie3019ef613a507023958bea27b1badc3b8950279