This function takes a commit list and returns a commit list. The
returned list is built by modifying the original list. Thus the caller
should not use the original list again (and after the next commit fixes
a memory leak, it must not).

Change the function signature so that it takes a **list and has void
return type. That should make it harder to misuse this function.

While we're here, document this function.
Signed-off-by: NMartin Ågren <martin.agren@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>