graph: rename `new_mapping` to `old_mapping`

The change I'm about to make requires being able to inspect the mapping
array that was used to render the last GRAPH_COLLAPSING line while
rendering a GRAPH_COMMIT line. The `new_mapping` array currently exists
as a pre-allocated space for computing the next `mapping` array during
`graph_output_collapsing_line()`, but we can repurpose it to let us see
the previous `mapping` state.

To support this use it will make more sense if this array is named
`old_mapping`, as it will contain the mapping data for the previous line
we rendered, at the point we're rendering a commit line.

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

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