cocci: add and apply free_commit_list() rules

cocci: add and apply free_commit_list() rules

Add and apply coccinelle rules to remove "if (E)" before
"free_commit_list(E)", the function can accept NULL, and further
change cases where "E = NULL" followed to also be unconditionally.

The code changes in this commit were entirely made by the coccinelle
rule being added here, and applied with:

    make contrib/coccinelle/free.cocci.patch
    patch -p1 <contrib/coccinelle/free.cocci.patch

The only manual intervention here is that the the relevant code in
commit.c has been manually re-indented.

Suggested-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>