run-command: introduce CHILD_PROCESS_INIT

Most struct child_process variables are cleared using memset first after
declaration.  Provide a macro, CHILD_PROCESS_INIT, that can be used to
initialize them statically instead.  That's shorter, doesn't require a
function call and is slightly more readable (especially given that we
already have STRBUF_INIT, ARGV_ARRAY_INIT etc.).

Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/replace.c b/builtin/replace.c
index 294b61b..d2aac64 100644
--- a/builtin/replace.c
+++ b/builtin/replace.c
@@ -197,7 +197,7 @@
 static void export_object(const unsigned char *sha1, enum object_type type,
 			  int raw, const char *filename)
 {
-	struct child_process cmd = { NULL };
+	struct child_process cmd = CHILD_PROCESS_INIT;
 	int fd;
 
 	fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0666);
@@ -234,7 +234,7 @@
 
 	if (!raw && type == OBJ_TREE) {
 		const char *argv[] = { "mktree", NULL };
-		struct child_process cmd = { argv };
+		struct child_process cmd = CHILD_PROCESS_INIT;
 		struct strbuf result = STRBUF_INIT;
 
 		cmd.argv = argv;