patch 8.1.2396: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/usercmd.c b/src/usercmd.c
index b48f157..5db6f48 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -392,7 +392,7 @@
     long	a;
     garray_T	*gap;
 
-    /* In cmdwin, the alternative buffer should be used. */
+    // In cmdwin, the alternative buffer should be used.
     gap =
 #ifdef FEAT_CMDWIN
 	(cmdwin_type != 0 && get_cmdline_type() == NUL) ?