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>

builtin/merge.c

@@ -1273,7 +1273,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	int best_cnt = -1, merge_was_ok = 0, automerge_was_ok = 0;
 	struct commit_list *common = NULL;
 	const char *best_strategy = NULL, *wt_strategy = NULL;
-	struct commit_list *remoteheads, *p;
+	struct commit_list *remoteheads = NULL, *p;
 	void *branch_to_free;
 	int orig_argc = argc;
 
@@ -1752,6 +1752,10 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		ret = suggest_conflicts();
 
 done:
+	if (!automerge_was_ok) {
+		free_commit_list(common);
+		free_commit_list(remoteheads);
+	}
 	strbuf_release(&buf);
 	free(branch_to_free);
 	return ret;