pack-objects: plug leak in `read_stdin_packs()`
The `read_stdin_packs()` function added originally via 339bce27f4f (builtin/pack-objects.c: add '--stdin-packs' option, 2021-02-22) declares a `rev_info` struct but neglects to call `release_revisions()` on it before returning, creating the potential for a leak. The related change in 97ec43247c0 (pack-objects: declare 'rev_info' for '--stdin-packs' earlier, 2025-06-23) carried forward this oversight and did not address it. Ensure that we call `release_revisions()` appropriately to prevent a potential leak from this function. Note that in practice our `rev_info` here does not have a present leak, hence t5331 passes cleanly before this commit, even when built with SANITIZE=leak. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>