reduce_heads: fix memory leaks

We currently have seven callers of `reduce_heads(foo)`. Six of them do
not use the original list `foo` again, and actually, all six of those
end up leaking it.

Introduce and use `reduce_heads_replace(&foo)` as a leak-free version of
`foo = reduce_heads(foo)` to fix several of these. Fix the remaining
leaks using `free_commit_list()`.

While we're here, document `reduce_heads()` and mark it as `extern`.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -999,6 +999,7 @@ static struct commit_list *reduce_parents(struct commit *head_commit,
 
 	/* Find what parents to record by checking independent ones. */
 	parents = reduce_heads(remoteheads);
+	free_commit_list(remoteheads);
 
 	remoteheads = NULL;
 	remotes = &remoteheads;