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>

diff-lib.c

@@ -642,6 +642,7 @@ int do_diff_cache(const struct object_id *tree_oid, struct diff_options *opt)
 	if (diff_cache(&revs, tree_oid, NULL, 1))
 		exit(128);
 	clear_pathspec(&revs.prune_data);
+	release_revisions(&revs);
 	return 0;
 }