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/ex_docmd.c

@@ -356,9 +356,6 @@ static void	ex_folddo(exarg_T *eap);
 # define ex_nbstart		ex_ni
 #endif
 
-#ifndef FEAT_CMDHIST
-# define ex_history		ex_ni
-#endif
 #ifndef FEAT_JUMPLIST
 # define ex_jumps		ex_ni
 # define ex_clearjumps		ex_ni
@@ -985,7 +982,7 @@ do_cmdline(
 	if (next_cmdline == NULL)
 	{
 	    VIM_CLEAR(cmdline_copy);
-#ifdef FEAT_CMDHIST
+
 	    /*
 	     * If the command was typed, remember it for the ':' register.
 	     * Do this AFTER executing the command to make :@: work.
@@ -997,7 +994,6 @@ do_cmdline(
 		last_cmdline = new_last_cmdline;
 		new_last_cmdline = NULL;
 	    }
-#endif
 	}
 	else
 	{
@@ -4130,12 +4126,10 @@ set_one_cmd_context(
 	    xp->xp_pattern = arg;
 	    break;
 
-#if defined(FEAT_CMDHIST)
 	case CMD_history:
 	    xp->xp_context = EXPAND_HISTORY;
 	    xp->xp_pattern = arg;
 	    break;
-#endif
 #if defined(FEAT_PROFILE)
 	case CMD_syntime:
 	    xp->xp_context = EXPAND_SYNTIME;