*.h: move some *_INIT to designated initializers

Move *_INIT macros I'll use in a subsequent commits to designated
initializers. This isn't required for those follow-up changes, but
since next commits will change things in this area, let's use the
modern pattern over the old one while we're at it.

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

credential.h

@@ -128,7 +128,9 @@ struct credential {
 	char *path;
 };
 
-#define CREDENTIAL_INIT { STRING_LIST_INIT_DUP }
+#define CREDENTIAL_INIT { \
+	.helpers = STRING_LIST_INIT_DUP, \
+}
 
 /* Initialize a credential structure, setting all fields to empty. */
 void credential_init(struct credential *);

json-writer.h

@@ -64,7 +64,10 @@ struct json_writer
 	unsigned int pretty:1;
 };
 
-#define JSON_WRITER_INIT { STRBUF_INIT, STRBUF_INIT, 0, 0 }
+#define JSON_WRITER_INIT { \
+	.json = STRBUF_INIT, \
+	.open_stack = STRBUF_INIT, \
+}
 
 void jw_init(struct json_writer *jw);
 void jw_release(struct json_writer *jw);

run-command.h

@@ -141,7 +141,10 @@ struct child_process {
 	void *clean_on_exit_handler_cbdata;
 };
 
-#define CHILD_PROCESS_INIT { NULL, STRVEC_INIT, STRVEC_INIT }
+#define CHILD_PROCESS_INIT { \
+	.args = STRVEC_INIT, \
+	.env_array = STRVEC_INIT, \
+}
 
 /**
  * The functions: child_process_init, start_command, finish_command,

string-list.h

@@ -91,8 +91,8 @@ struct string_list {
 	compare_strings_fn cmp; /* NULL uses strcmp() */
 };
 
-#define STRING_LIST_INIT_NODUP { NULL, 0, 0, 0, NULL }
-#define STRING_LIST_INIT_DUP   { NULL, 0, 0, 1, NULL }
+#define STRING_LIST_INIT_NODUP { 0 }
+#define STRING_LIST_INIT_DUP   { .strdup_strings = 1 }
 
 /* General functions which work with both sorted and unsorted lists. */