patch 8.2.2922: computing array length is done in various ways

Problem:    Computing array length is done in various ways.
Solution:   Use ARRAY_LENGTH everywhere. (Ken Takata, closes #8305)
diff --git a/src/cmdhist.c b/src/cmdhist.c
index 1e7ae34..8bb3cb6 100644
--- a/src/cmdhist.c
+++ b/src/cmdhist.c
@@ -98,7 +98,7 @@
     static char_u compl[2] = { NUL, NUL };
     char *short_names = ":=@>?/";
     int short_names_count = (int)STRLEN(short_names);
-    int history_name_count = sizeof(history_names) / sizeof(char *) - 1;
+    int history_name_count = ARRAY_LENGTH(history_names) - 1;
 
     if (idx < short_names_count)
     {