patch 8.1.1823: command line history code is spread out

Problem:    Command line history code is spread out.
Solution:   Put the code in a new file. (Yegappan Lakshmanan, closes #4779)
            Also graduate the +cmdline_hist feature.

src/ops.c

@@ -1233,7 +1233,6 @@ do_execreg(
     if (regname == '_')
 	return OK;
 
-#ifdef FEAT_CMDHIST
     // use last command line
     if (regname == ':')
     {
@@ -1262,7 +1261,6 @@ do_execreg(
 	}
 	vim_free(p);
     }
-#endif
 #ifdef FEAT_EVAL
     else if (regname == '=')
     {