patch 8.2.1007: completion doesn't work after ":r ++arg !"

Problem:    Completion doesn't work after ":r ++arg !".
Solution:   Skip over "++arg". (Christian Brabandt, closes #6275,
            closes #6258)
diff --git a/src/cmdexpand.c b/src/cmdexpand.c
index 9508d66..63f0cc7 100644
--- a/src/cmdexpand.c
+++ b/src/cmdexpand.c
@@ -1146,6 +1146,15 @@
 	arg = skipwhite(arg);
     }
 
+    // Skip over ++argopt argument
+    if ((ea.argt & EX_ARGOPT) && *arg != NUL && STRNCMP(arg, "++", 2) == 0)
+    {
+	p = arg;
+	while (*p && !vim_isspace(*p))
+	    MB_PTR_ADV(p);
+	arg = skipwhite(p);
+    }
+
     // Check for '|' to separate commands and '"' to start comments.
     // Don't do this for ":read !cmd" and ":write !cmd".
     if ((ea.argt & EX_TRLBAR) && !usefilter)