NewThreeWay creates a new Policy based on ThreeWay using the provided ResolveFunc.

None is the no-op ResolveFunc.

Ours resolves conflicts by preferring changes from the Value currently being committed.

Theirs resolves conflicts by preferring changes in the current HEAD.

ThreeWay attempts a three-way merge between two _candidate_ values that have both changed with respect to a common _parent_ value.

ErrMergeConflict indicates that a merge attempt failed and must be resolved manually for the provided reason.

Policy functors are used to merge two values (a and b) against a common ancestor.

ResolveFunc is the type for custom merge-conflict resolution callbacks.