*.[ch] refactoring: make use of the FREE_AND_NULL() macro

Replace occurrences of `free(ptr); ptr = NULL` which weren't caught by
the coccinelle rule. These fall into two categories:

 - free/NULL assignments one after the other which coccinelle all put
   on one line, which is functionally equivalent code, but very ugly.

 - manually spotted occurrences where the NULL assignment isn't right
   after the free() call.

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

help.c

@@ -267,9 +267,8 @@ static void add_cmd_list(struct cmdnames *cmds, struct cmdnames *old)
 
 	for (i = 0; i < old->cnt; i++)
 		cmds->names[cmds->cnt++] = old->names[i];
-	free(old->names);
+	FREE_AND_NULL(old->names);
 	old->cnt = 0;
-	old->names = NULL;
 }
 
 /* An empirically derived magic number */