hg-fast-export.py: do not sort merge commit parents

In a merge commit, the first parent is always the same parent that
would be recorded if the commit were not a merge and the other
parent(s) record the commit(s) being merged in.

Preserving this order is important so that log --first-parent works
properly and also so that the merge history is not distorted by an
incorrect permutation of the DAG.

Remove the code that sorts the merge parents based on node id so
that the correct DAG order is preserved.