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.

src/os_win32.c

@@ -4767,6 +4767,9 @@ mch_call_shell(
     int		tmode = cur_tmode;
     WCHAR	szShellTitle[512];
 
+#ifdef FEAT_JOB_CHANNEL
+    ch_log(NULL, "executing shell command: %s", cmd);
+#endif
     // Change the title to reflect that we are in a subshell.
     if (GetConsoleTitleW(szShellTitle, ARRAY_LENGTH(szShellTitle) - 4) > 0)
     {