Relax topological restrictions with ctrl+select (and 2 selections)

[ianhattendorf]

Description

Use case: if I amend a commit, I might want to compare the amended commit with the original (on the remote). With "gitlens.graph.multiselect": "topological" (the default), this isn't possible. I also occasionally compare other commits that aren't topological. Still keep the topological restriction when more than two commits are selected.