traverse_bitmap_commit_list(): don't free result

traverse_bitmap_commit_list(): don't free result

Since it was introduced in fff42755ef (pack-bitmap: add
support for bitmap indexes, 2013-12-21), this function has
freed the result after traversing it. That is an artifact of
the early days of the bitmap code, when we had a single
static "struct bitmap_index". Back then, it was intended
that you would do:

  prepare_bitmap_walk(&revs);
  traverse_bitmap_commit_list(&revs);

Since the actual bitmap_index struct was totally behind the
scenes, it was convenient for traverse_bitmap_commit_list()
to clean it up, clearing the way for another traversal.

But since 3ae5fa0768 (pack-bitmap: remove bitmap_git global
variable, 2018-06-07), the caller explicitly manages the
bitmap_index struct itself, like this:

  b = prepare_bitmap_walk(&revs);
  traverse_bitmap_commit_list(b, &revs);
  free_bitmap_index(b);

It no longer makes sense to auto-free the result after the
traversal. If you want to do another traversal, you'd just
create a new bitmap_index. And while nobody tries to call
traverse_bitmap_commit_list() twice, the fact that it throws
away the result might be surprising, and is better avoided.

Note that in the "old" way it was possible for two walks to
amortize the cost of opening the on-disk .bitmap file (since
it was stored in the global bitmap_index), but we lost that
in 3ae5fa0768. However, no code actually does this, so it's
not worth addressing now. The solution might involve a new:

  reset_bitmap_walk(b, &revs);

call. Or we might even attach the bitmap data to its
matching packed_git struct, so that multiple
prepare_bitmap_walk() calls could use it. That can wait
until somebody actually has need of the optimization (and
until then, we'll do the correct, unsurprising thing).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>