line-log: fix leak when rewriting commit parents

In `process_ranges_merge_commit()` we try to figure out which of the
parents can be blamed for the given line changes. When we figure out
that none of the files in the line-log have changed we assign the
complete blame to that commit and rewrite the parents of the current
commit to only use that single parent.

This is done via `commit_list_append()`, which is misleadingly _not_
appending to the list of parents. Instead, we overwrite the parents with
the blamed parent. This makes us lose track of the old pointers,
creating a memory leak.

Fix this issue by freeing the parents before we overwrite them.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

line-log.c

@@ -1237,6 +1237,7 @@ static int process_ranges_merge_commit(struct rev_info *rev, struct commit *comm
 			 * don't follow any other path in history
 			 */
 			add_line_range(rev, parents[i], cand[i]);
+			free_commit_list(commit->parents);
 			commit_list_append(parents[i], &commit->parents);
 
 			ret = 0;