Fix init-branch selecting changeset later than first commit in branch as root

When I run init-branch on some branches I encountered errors. The problem seemed to be that gittfs picked a changeset later than the first changeset in the branch to be created. Actually it even picked a changeset that was on the branch that was suppose to be created which made it impossible to create the branch itself. The extra condition in LastOrDefault did te trick for me.
I also changed the error message a bit to hopefully make it more clear.