Commands block text overwritten when using formatted text

*Verification builds:* 1.2.5 (530691)

*Summary:* When using formatted (§) text in a command block, going in and changing the command later by using the left/right arrow key will cause the command to break.

*Impact:* The command is lost, or changed in unexpected ways.

*Repro Steps:*
1. Create a command block
2. Type say Hello §aworld
3. Use the left arrow key to change Hello to Good Morning

*Observed Results:*
The command will become mangled.

*Expected Results:*
The command should be editable.

**Screenshots/Videos attached: Yes (https://www.youtube.com/watch?v=FoNT0tz0PyE&feature=youtu.be)

*Notes:*