patch 8.1.1913: not easy to compute the space on the command line

Problem:    Not easy to compute the space on the command line.
Solution:   Add v:echospace. (Daniel Hahler, closes #4732)
diff --git a/src/option.c b/src/option.c
index 6d3a059..215d48a 100644
--- a/src/option.c
+++ b/src/option.c
@@ -10881,6 +10881,9 @@
     sc_col = Columns;
     ru_col = Columns;
 #endif
+#ifdef FEAT_EVAL
+    set_vim_var_nr(VV_ECHOSPACE, sc_col - 1);
+#endif
 }
 
 #if defined(FEAT_PYTHON) || defined(FEAT_PYTHON3) || defined(PROTO)