patch 8.2.4742: there is no way to start logging very early in startup

Problem:    There is no way to start logging very early in startup.
Solution:   Add the --log argument.  Include the date in the start message in
            the log file.  Avoid a duplicate message when forking.  Log an
            executed shell command.
diff --git a/src/os_unix.c b/src/os_unix.c
index 826c9c0..d0675e8 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -5480,6 +5480,9 @@
     char_u	*cmd,
     int		options)	// SHELL_*, see vim.h
 {
+#ifdef FEAT_JOB_CHANNEL
+    ch_log(NULL, "executing shell command: %s", cmd);
+#endif
 #if defined(FEAT_GUI) && defined(FEAT_TERMINAL)
     if (gui.in_use && vim_strchr(p_go, GO_TERMINAL) != NULL)
 	return mch_call_shell_terminal(cmd, options);