revisions API: have release_revisions() release "topo_walk_info"

Refactor the existing reset_topo_walk() into a thin wrapper for a
release_revisions_topo_walk_info() + resetting the member to "NULL",
and call release_revisions_topo_walk_info() from release_revisions().

This fixes memory leaks that have been with us ever since
"topo_walk_info" was added to revision.[ch] in
f0d9cc4196a (revision.c: begin refactoring --topo-order logic,
2018-11-01).

Due to various other leaks this makes no tests pass in their entirety,
but e.g. before this running this on git.git:

    ./git -P log --pretty=tformat:"%P   %H | %s" --parents --full-history --topo-order -3 -- README.md

Would report under SANITIZE=leak:

    SUMMARY: LeakSanitizer: 531064 byte(s) leaked in 6 allocation(s).

Now we'll free all of that memory.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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