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gt bco <trunk_name>

?

The problem is, that you have to remember the name for the current project, and there is no convenient abbreviation. You could also use

gt bco <upmost branch>

, yet there is

gt bt

. You could also use

gt bco <bottommost branch>

, yet there is

gt bb

. As soon as you have multiple branch-off points from the trunk branch, I find it quite common needing to be on the trunk branch to restack and resubmit the whole tree after there is an incoming update.