Fix memory leak in traverse_commit_list

Fix memory leak in traverse_commit_list

If we were listing objects too then the objects were buffered in an
array only reachable from a stack allocated structure.  When this
function returns that array would be leaked as nobody would have
a reference to it anymore.

Historically this hasn't been a problem as the primary user of
traverse_commit_list() (the noble git-rev-list) would terminate
as soon as the function was finished, thus allowing the operating
system to cleanup memory.  However we have been leaking this data
in git-pack-objects ever since that program learned how to run the
revision listing internally, rather than relying on reading object
names from git-rev-list.

To better facilitate reuse of traverse_commit_list during other
builtin tools (such as git-fetch) we shouldn't leak temporary memory
like this and instead we need to clean up properly after ourselves.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>