builtin/merge-base: free commit lists

builtin/merge-base: free commit lists

In several functions, we iterate through a commit list by assigning
`result = result->next`. As a consequence, we lose the original pointer
and eventually leak the list.

Rewrite the loops so that we keep the original pointers, then call
`free_commit_list()`. Various alternatives were considered:

1) Use `UNLEAK(result)` before the loop. Simple change, but not very
pretty. These would definitely be new lows among our usages of UNLEAK.
2) Use `pop_commit()` when looping. Slightly less simple change, but it
feels slightly preferable to first display the list, then free it.
3) As in this patch, but with `UNLEAK()` instead of freeing. We'd still
go through all the trouble of refactoring the loop, and because it's not
super-obvious that we're about to exit, let's just free the lists -- it
probably doesn't affect the runtime much.

In `handle_independent()` we can drop `result` while we're here and
reuse the `revs`-variable instead. That matches several other users of
`reduce_heads()`. The memory-leak that this hides will be addressed in
the next commit.

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