bisect: change calling-convention of `find_bisection()`

bisect: change calling-convention of `find_bisection()`

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