fsmonitor: use internal argv_array of struct child_process

Avoid magic array sizes and indexes by constructing the fsmonitor
command line using the embedded argv_array of the child_process.  The
resulting code is shorter and easier to extend.

Getting rid of the snprintf() calls is a bonus -- even though the
buffers were big enough here to avoid truncation -- as it makes auditing
the remaining callers easier.

Inspired-by: Jeff King <peff@peff.net>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsmonitor.c

@@ -97,19 +97,13 @@ void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate)
 static int query_fsmonitor(int version, uint64_t last_update, struct strbuf *query_result)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
-	char ver[64];
-	char date[64];
-	const char *argv[4];
 
-	if (!(argv[0] = core_fsmonitor))
+	if (!core_fsmonitor)
 		return -1;
 
-	snprintf(ver, sizeof(version), "%d", version);
-	snprintf(date, sizeof(date), "%" PRIuMAX, (uintmax_t)last_update);
-	argv[1] = ver;
-	argv[2] = date;
-	argv[3] = NULL;
-	cp.argv = argv;
+	argv_array_push(&cp.args, core_fsmonitor);
+	argv_array_pushf(&cp.args, "%d", version);
+	argv_array_pushf(&cp.args, "%" PRIuMAX, (uintmax_t)last_update);
 	cp.use_shell = 1;
 	cp.dir = get_git_work_tree();