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>

contrib/coccinelle/free.cocci

@@ -2,13 +2,21 @@
 expression E;
 @@
 - if (E)
+(
   free(E);
+|
+  free_commit_list(E);
+)
 
 @@
 expression E;
 @@
 - if (!E)
+(
   free(E);
+|
+  free_commit_list(E);
+)
 
 @@
 expression E;
@@ -16,3 +24,22 @@ expression E;
 - free(E);
 + FREE_AND_NULL(E);
 - E = NULL;
+
+@@
+expression E;
+@@
+- if (E)
+- {
+  free_commit_list(E);
+  E = NULL;
+- }
+
+@@
+expression E;
+statement S;
+@@
+- if (E) {
++ if (E)
+  S
+  free_commit_list(E);
+- }