*.c *_init(): define in terms of corresponding *_INIT macro

Change the common patter in the codebase of duplicating the
initialization logic between an *_INIT macro and a
corresponding *_init() function to use the macro as the canonical
source of truth.

Now we no longer need to keep the function up-to-date with the macro
version. This implements a suggestion by Jeff King who found that
under -O2 [1] modern compilers will init new version in place without
the extra copy[1]. The performance of a single *_init() won't matter
in most cases, but even if it does we're going to be producing
efficient machine code to perform these operations.

1. https://lore.kernel.org/git/YNyrDxUO1PlGJvCn@coredump.intra.peff.net/

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

credential.c

@@ -10,8 +10,8 @@
 
 void credential_init(struct credential *c)
 {
-	memset(c, 0, sizeof(*c));
-	c->helpers.strdup_strings = 1;
+	struct credential blank = CREDENTIAL_INIT;
+	memcpy(c, &blank, sizeof(*c));
 }
 
 void credential_clear(struct credential *c)

json-writer.c

@@ -3,10 +3,8 @@
 
 void jw_init(struct json_writer *jw)
 {
-	strbuf_init(&jw->json, 0);
-	strbuf_init(&jw->open_stack, 0);
-	jw->need_comma = 0;
-	jw->pretty = 0;
+	struct json_writer blank = JSON_WRITER_INIT;
+	memcpy(jw, &blank, sizeof(*jw));;
 }
 
 void jw_release(struct json_writer *jw)

run-command.c

@@ -11,9 +11,8 @@
 
 void child_process_init(struct child_process *child)
 {
-	memset(child, 0, sizeof(*child));
-	strvec_init(&child->args);
-	strvec_init(&child->env_array);
+	struct child_process blank = CHILD_PROCESS_INIT;
+	memcpy(child, &blank, sizeof(*child));
 }
 
 void child_process_clear(struct child_process *child)

strbuf.c

@@ -52,8 +52,8 @@ char strbuf_slopbuf[1];
 
 void strbuf_init(struct strbuf *sb, size_t hint)
 {
-	sb->alloc = sb->len = 0;
-	sb->buf = strbuf_slopbuf;
+	struct strbuf blank = STRBUF_INIT;
+	memcpy(sb, &blank, sizeof(*sb));
 	if (hint)
 		strbuf_grow(sb, hint);
 }

strmap.c

@@ -25,7 +25,8 @@ static struct strmap_entry *find_strmap_entry(struct strmap *map,
 
 void strmap_init(struct strmap *map)
 {
-	strmap_init_with_options(map, NULL, 1);
+	struct strmap blank = STRMAP_INIT;
+	memcpy(map, &blank, sizeof(*map));
 }
 
 void strmap_init_with_options(struct strmap *map,

strvec.c

@@ -6,9 +6,8 @@ const char *empty_strvec[] = { NULL };
 
 void strvec_init(struct strvec *array)
 {
-	array->v = empty_strvec;
-	array->nr = 0;
-	array->alloc = 0;
+	struct strvec blank = STRVEC_INIT;
+	memcpy(array, &blank, sizeof(*array));
 }
 
 static void strvec_push_nodup(struct strvec *array, const char *value)