parsers: simplify the code computing the phases We recently simplified the code computing the heads of a repo. This patch uses the same simplification for phase computation. We use index_get_parents instead of replicating its code.

parsers: move index_get_parents's declaration higher index_get_parents needs to be used in the phase computation code so we need to move its declaration higher to be able to call it. It cannot be moved any higher than that so we won't have any more patch doing the same thing.