Allow Dataset HEAD to be set to non-fastforward commits (#2271)

The Dataset.Commit() code pathway still enforces fast-forward-only
behavior, but a new SetHead() method allows the HEAD of a Dataset to
be forced to any other Commit.

noms sync detects the case where the source Commit is not a descendent
of the provided sink Dataset's HEAD and uses the new API to force the
sink to the desired new Commit, printing out the now-abandoned old
HEAD.

Fixes #2240

go/datas/commit.go

@@ -122,6 +122,6 @@ func IsCommitType(t *types.Type) bool {
 	return types.IsSubtype(valueCommitType, t)
 }
 
-func isRefOfCommitType(t *types.Type) bool {
+func IsRefOfCommitType(t *types.Type) bool {
 	return t.Kind() == types.RefKind && IsCommitType(getRefElementType(t))
 }