This fixes a minor memory leak (detected by LeakSanitizer) in git merge.

To reproduce (with an ASAN build):

```
mkdir test
cd test
git init
echo x > x.txt
git add .
git commit -m "WIP"
git checkout -b dev
echo y > x.txt
git add .
git commit -m "WIP"
git checkout main
echo z > x.txt
git add .
git commit -m "WIP"
echo a > x.txt
git add .
git merge dev
```

The fix is to call free_commit_list(merge_bases) when an error occurs.

Signed-off-by: Kevin Backhouse <kevinbackhouse@github.com>

merge-ort-wrappers.c

@@ -54,8 +54,10 @@ int merge_ort_recursive(struct merge_options *opt,
 	struct tree *head = repo_get_commit_tree(opt->repo, side1);
 	struct merge_result tmp;
 
-	if (unclean(opt, head))
+	if (unclean(opt, head)) {
+		free_commit_list(merge_bases);
 		return -1;
+	}
 
 	memset(&tmp, 0, sizeof(tmp));
 	merge_incore_recursive(opt, merge_bases, side1, side2, &tmp);

merge-ort.c

@@ -5070,8 +5070,10 @@ static void merge_ort_internal(struct merge_options *opt,
 		opt->branch1 = "Temporary merge branch 1";
 		opt->branch2 = "Temporary merge branch 2";
 		merge_ort_internal(opt, NULL, prev, next, result);
-		if (result->clean < 0)
+		if (result->clean < 0) {
+			free_commit_list(merge_bases);
 			return;
+		}
 		opt->branch1 = saved_b1;
 		opt->branch2 = saved_b2;
 		opt->priv->call_depth--;