commit: rename `free_commit_list()` to conform to coding guidelines

commit: rename `free_commit_list()` to conform to coding guidelines

Our coding guidelines say that:

  Functions that operate on `struct S` are named `S_<verb>()` and should
  generally receive a pointer to `struct S` as first parameter.

While most of the functions related to `struct commit_list` already
follow that naming schema, `free_commit_list()` doesn't.

Rename the function to address this and adjust all of its callers. Add a
compatibility wrapper for the old function name to ease the transition
and avoid any semantic conflicts with in-flight patch series. This
wrapper will be removed once Git 2.53 has been released.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>