Allow triggering a child plan to override parents git head

Bamboo plans can be triggered by other plans, and in doing so the child plan will assume that it should use the @head value from the parent plan repository. Subsequent triggering of the "child" plan, not by the parent but by an update to a repository, will not detect the latest @head, but use the parents last value which may be out of date. Some customer workflows may require both parent/child relationships, and also arbitrary running of a child plan as well.

To improve this behaviour, if triggered by a parent plan, use the parents @head, but if a child plan is triggered by another method, enable a choice to find the @head as set in the current repo state, not the parents last.