Issue Description:

Since Swift 5.1, it is no longer possible to go through the command history in the REPL by pressing the up/down arrow keys. It is only possible to recall the most recent command.

Steps to reproduce:

1. Open a terminal

2. Type in swift and press return to start the REPL

3. Type in the following:

   func foo() {}
   func bar() {} 

4. Press the up arrow key twice

Expected results:

The current input should be

func foo() {} 

Actual results:

The current line input is

func bar() {} 

Further notes:

Pressing the down arrow after pressing the up arrow results in a command from a previous sessions being shown.