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/pager.c b/pager.c
index 8b5cbc5..d0e4bc8 100644
--- a/pager.c
+++ b/pager.c
@@ -18,7 +18,7 @@
  */
 
 static const char *pager_argv[] = { NULL, NULL };
-static struct child_process pager_process;
+static struct child_process pager_process = CHILD_PROCESS_INIT;
 
 static void wait_for_pager(void)
 {