graph: reuse `find_new_column_by_commit()`

I will shortly be making some changes to
`graph_insert_into_new_columns()` and so am trying to simplify it. One
possible simplification is that we can extract the loop for finding the
element in `new_columns` containing the given commit.

`find_new_column_by_commit()` contains a very similar loop but it
returns a `struct column *` rather than an `int` offset into the array.
Here I'm introducing a version that returns `int` and using that in
`graph_insert_into_new_columns()` and `graph_output_post_merge_line()`.

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

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