patch 7.4.2015
Problem:    When a file gets a name when writing it 'acd' is not effective.
            (Dan Church)
Solution:   Invoke DO_AUTOCHDIR after writing the file. (Allen Haim, closes
            #777, closes #803)  Add test_autochdir() to enable 'acd' before
            "starting" is reset.
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 5900c86..d93a7ec 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -3059,6 +3059,7 @@
     char_u	*browse_file = NULL;
 #endif
     buf_T	*alt_buf = NULL;
+    int		name_was_missing;
 
     if (not_writing())		/* check 'write' option */
 	return FAIL;
@@ -3226,6 +3227,8 @@
 #endif
 	}
 
+	name_was_missing = curbuf->b_ffname == NULL;
+
 	retval = buf_write(curbuf, ffname, fname, eap->line1, eap->line2,
 				 eap, eap->append, eap->forceit, TRUE, FALSE);
 
@@ -3239,7 +3242,12 @@
 		redraw_tabline = TRUE;
 #endif
 	    }
-	    /* Change directories when the 'acd' option is set. */
+	}
+
+	/* Change directories when the 'acd' option is set and the file name
+	 * got changed or set. */
+	if (eap->cmdidx == CMD_saveas || name_was_missing)
+	{
 	    DO_AUTOCHDIR
 	}
     }