lib/traverse: Fix minor memory leak

lib/traverse: Fix minor memory leak

I was trying to check something with `-fsanitize=address`
and it warned about this memory leak. It's...subtle, basically
we were leaking when the same commit was added to the hash table.

But unfortunately fixing that then complicates ownership
over the return value; what we really want to use here is
`g_hash_table_steal_all_keys` but RHEL 9.4 is still rocking
`glib2-2.68.4` so we can't use it.

(Rust would mean we wouldn't have leaked anything here in the
 first place...)

Signed-off-by: Colin Walters <walters@verbum.org>