patch 8.2.2806: Vim9: using "++nr" as a command might not work

Problem:    Vim9: using "++nr" as a command might not work.
Solution:   Do not recognize "++" and "--" in a following line as addition or
            subtraction.
diff --git a/src/eval.c b/src/eval.c
index fbf4f57..97a00f7 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -2856,12 +2856,15 @@
 
 	// "." is only string concatenation when scriptversion is 1
 	// "+=", "-=" and "..=" are assignments
+	// "++" and "--" on the next line are a separate command.
 	p = eval_next_non_blank(*arg, evalarg, &getnext);
 	op = *p;
 	concat = op == '.' && (*(p + 1) == '.' || current_sctx.sc_version < 2);
 	if ((op != '+' && op != '-' && !concat) || p[1] == '='
 					       || (p[1] == '.' && p[2] == '='))
 	    break;
+	if (getnext && (op == '+' || op == '-') && p[0] == p[1])
+	    break;
 
 	evaluate = evalarg == NULL ? 0 : (evalarg->eval_flags & EVAL_EVALUATE);
 	oplen = (concat && p[1] == '.') ? 2 : 1;