patch 8.0.0218: no completion for :cexpr and similar commands

Problem:    No command line completion for :cexpr, :cgetexpr, :caddexpr, etc.
Solution:   Make completion work. (Yegappan Lakshmanan)  Add a test.

src/ex_docmd.c

@@ -4109,6 +4109,12 @@ set_one_cmd_context(
 	case CMD_echoerr:
 	case CMD_call:
 	case CMD_return:
+	case CMD_cexpr:
+	case CMD_caddexpr:
+	case CMD_cgetexpr:
+	case CMD_lexpr:
+	case CMD_laddexpr:
+	case CMD_lgetexpr:
 	    set_context_for_expression(xp, arg, ea.cmdidx);
 	    break;
 

src/testdir/test_cmdline.vim

@@ -25,6 +25,34 @@ func Test_complete_wildmenu()
   set nowildmenu
 endfunc
 
+func Test_expr_completion()
+  if !(has('cmdline_compl') && has('eval'))
+    return
+  endif
+  for cmd in [
+	\ 'let a = ',
+	\ 'if',
+	\ 'elseif',
+	\ 'while',
+	\ 'for',
+	\ 'echo',
+	\ 'echon',
+	\ 'execute',
+	\ 'echomsg',
+	\ 'echoerr',
+	\ 'call',
+	\ 'return',
+	\ 'cexpr',
+	\ 'caddexpr',
+	\ 'cgetexpr',
+	\ 'lexpr',
+	\ 'laddexpr',
+	\ 'lgetexpr']
+    call feedkeys(":" . cmd . " getl\<Tab>\<Home>\"\<CR>", 'xt')
+    call assert_equal('"' . cmd . ' getline(', getreg(':'))
+  endfor
+endfunc
+
 func Test_getcompletion()
   if !has('cmdline_compl')
     return

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    218,
 /**/
     217,
 /**/