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>