tree-walk: report recursion counts

The traverse_trees() method recursively walks through trees, but also
prunes the tree-walk based on a callback. Some callers, such as
unpack_trees(), are quite complicated and can have wildly different
performance between two different commands.

Create constants that count these values and then report the results at
the end of a process. These counts are cumulative across multiple "root"
instances of traverse_trees(), but they provide reproducible values for
demonstrating improvements to the pruning algorithm when possible.

This change is modeled after a similar statistics reporting in 42e50e78
(revision.c: add trace2 stats around Bloom filter usage, 2020-04-06).

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

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