merge: fix memory leaks in cmd_merge()

There were two commit_lists created in cmd_merge() that were only
conditionally free()'d.  Add a quick conditional call to
free_commit_list() for each of them at the end of the function.

Testing this commit against t6404 under valgrind shows that this patch
fixes the following two leaks:

    16 bytes in 1 blocks are definitely lost in loss record 16 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 0x47FC93: reduce_parents (merge.c:1114)
       by 0x4801EE: collect_parents (merge.c:1214)
       by 0x480B56: cmd_merge (merge.c:1465)
       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)

    8 (16 direct, 32 indirect) bytes in 1 blocks are definitely lost in \
    loss record 61 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 0x52A8F2: commit_list_insert_by_date (commit.c:620)
       by 0x5270AC: get_merge_bases_many_0 (commit-reach.c:413)
       by 0x52716C: repo_get_merge_bases (commit-reach.c:438)
       by 0x480E5A: cmd_merge (merge.c:1520)
       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)

There are still 3 leaks in chdir_notify_register() after this, but
chdir_notify_register() has been brought up on the list before and folks
were not a fan of fixing those, so I'm not touching them.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
