graph: reduce duplication in `graph_insert_into_new_columns()`

I will shortly be making some changes to this function and so am trying
to simplify it. It currently contains some duplicated logic; both
branches the function can take assign the commit's column index into
the `mapping` array and increment `mapping_index`.

Here I change the function so that the only conditional behaviour is
that it appends the commit to `new_columns` if it's not present. All
manipulation of `mapping` now happens on a single code path.

Signed-off-by: James Coglan <jcoglan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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