revisions API users: use release_revisions() for "prune_data" users

revisions API users: use release_revisions() for "prune_data" users

Use release_revisions() for users of "struct rev_list" that reach into
the "struct rev_info" and clear the "prune_data" already.

In a subsequent commit we'll teach release_revisions() to clear this
itself, but in the meantime let's invoke release_revisions() here to
clear anything else we may have missed, and for reasons of having
consistent boilerplate.

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