Smart multi-commit selection in the graph

[justinrobots]

Description

When shift-clicking multiple commits in the graph, only select commits in the same branch.

In the example below, only the blue commit nodes would be selected when selecting "Bumps to v16.3.2" then shift-clicking "Bumps to v16.3.1", not the purple ones.

ctrl-clicking additional commits should continue to add commits to the selection.