leak_pending: use `object_array_clear()`, not `free()`

leak_pending: use `object_array_clear()`, not `free()`

Setting `leak_pending = 1` tells `prepare_revision_walk()` not to
release the `pending` array, and makes that the caller's responsibility.
See 4a43d374f (revision: add leak_pending flag, 2011-10-01) and
353f5657a (bisect: use leak_pending flag, 2011-10-01).

Commit 1da1e07c8 (clean up name allocation in prepare_revision_walk,
2014-10-15) fixed a memory leak in `prepare_revision_walk()` by
switching from `free()` to `object_array_clear()`. However, where we use
the `leak_pending`-mechanism, we're still only calling `free()`.

Use `object_array_clear()` instead. Copy some helpful comments from
353f5657a to the other callers that we update to clarify the memory
responsibilities, and to highlight that the commits are not affected
when we clear the array -- it is indeed correct to both tidy up the
commit flags and clear the object array.

Document `leak_pending` in revision.h to help future users get this
right.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>