revision: trace topo-walk statistics

We trace statistics about the effectiveness of changed-path Bloom
filters since 42e50e78 (revision.c: add trace2 stats around Bloom
filter usage, 2020-04-06). Add similar tracing for the topo-walk
algorithm that uses generation numbers to limit the walk size.

This information can help investigate and describe benefits to
heuristics and other changes.

The information that is printed is in JSON format and can be formatted
nicely to present as follows:

    {
	"count_explort_walked":2603,
	"count_indegree_walked":2603,
	"count_topo_walked":473
    }

Each of these values count the number of commits are visited by each of
the three "stages" of the topo-walk as detailed in b4542418 (revision.c:
generation-based topo-order algorithm, 2018-11-01).

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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