[xbuild] Return unescaped path from TaskItem.GetMetadata ("FullPath")

This is in accordance with the behavior in .NET. No matter what string you feed
the ctor (escaped or unescaped), GetMetadata("FullPath") always returns the
unescaped version.

This also fixes the monodevelop build of current master, which was broken on my
Ubuntu Trusty box since 6d7e812d709e182810d64c51a4e2d1af25d8adfa.