merge-ort: fix memory leak in merge_ort_internal()

merge-ort: fix memory leak in merge_ort_internal()

The documentation for merge_incore_recursive(), modelled after
merge_recursive(), notes that

   merge_bases will be consumed (emptied) so make a copy if you need it

However, in merge_ort_internal() (which merge_incore_recursive() calls),
it runs

   merged_merge_bases = pop_commit(&merge_bases);
   ...
   for (iter = merge_bases; iter; iter = iter->next) {
      ...
   }

In other words, it only consumes the *first* entry of merge_bases, and
the rest it iterates through.  If it iterated through all of them, the
caller could be responsible for free'ing the memory.  If it consumed all
of them, the current documentation would be correct and the callers
would need to do nothing.  The current middle ground makes it impossible
for callers to avoid memory leaks, since any attempt to use the
merge_bases it passes in would result in a use-after-free.

It turns out this part of the code was copied from merge-recursive.c,
which has had the same bug for 15.5 years.  However, since we are trying
to keep merge-recursive.c stable as we sunset it, let's just fix the
leak in in merge_ort_internal() by having it actually consume all the
elements of the merge_bases commit_list.

Testing this commit against t6404 (the first testcase specifically
about recursive merges) under valgrind shows that this patch fixes
the following leak:

    32 (16 direct, 16 indirect) bytes in 1 blocks are definitely lost \
    in loss record 49 of 126
       at 0x484086F: malloc (vg_replace_malloc.c:380)
       by 0x69FFEB: do_xmalloc (wrapper.c:41)
       by 0x6A0073: xmalloc (wrapper.c:62)
       by 0x52A72D: commit_list_insert (commit.c:556)
       by 0x47EC86: try_merge_strategy (merge.c:751)
       by 0x48143B: cmd_merge (merge.c:1679)
       by 0x40686E: run_builtin (git.c:464)
       by 0x406C51: handle_builtin (git.c:716)
       by 0x406E96: run_argv (git.c:783)
       by 0x40730A: cmd_main (git.c:914)
       by 0x4E7DFA: main (common-main.c:56)

Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>