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/notes.c b/builtin/notes.c
index 820c341..c25a412 100644
--- a/builtin/notes.c
+++ b/builtin/notes.c
@@ -122,12 +122,11 @@
 {
 	const char *show_args[5] =
 		{"show", "--stat", "--no-notes", sha1_to_hex(object), NULL};
-	struct child_process show;
+	struct child_process show = CHILD_PROCESS_INIT;
 	struct strbuf buf = STRBUF_INIT;
 	struct strbuf cbuf = STRBUF_INIT;
 
 	/* Invoke "git show --stat --no-notes $object" */
-	memset(&show, 0, sizeof(show));
 	show.argv = show_args;
 	show.no_stdin = 1;
 	show.out = -1;