Allow comparisons between Git timeline entries

[eamodio]

Ideally we should be able to multi-select timeline entries and compare the 2 ends of the selection. But even without multi-select we should allow the Select for Compare/Compare with Selected mechanism to provide a similar behavior.

[eamodio]

I've added support for the Select for Compare / Compare with Selected mechanism. And opened #117261 to track multi-select support.

[eamodio]

To verify:

1. Right-click on a Git timeline entry and choose Select for Compare
2. Right-click on another Git timeline entry and choose Compare with Selected
3. A diff should open between the chosen entries -- FYI the Select for Compare is always the left side of the diff.