branch track: track a whole untracked stack

[abhinav]

Scenario:

git checkout main
git checkout -b feat1
git checkout -b feat2

At that point, we'd like gs branch track or an equivalent command to track main -> feat1 -> feat2, not main -> feat2.
This would have to be opt-in to handle the case where the intermediate branch is untracked on purpose.

Possibilities:

• gs downstack track
• gs branch track --all