--code doesn't work with --check or --diff

[ajeetdsouza]

Describe the bug

• --code is supposed to take the code as an input string
• --check is supposed to change the exit code based on whether edits were made while formatting
• --diff is supposed to only output a diff, instead of the entire formatted source

What I found was the the --check and --diff flags did not work when passing input using --code; it behaved as if the extra flags hadn't been passed at all.

To Reproduce Steps to reproduce the behavior:

1. Run this command (notice the double space after =):

black --check --code 'a =  5'

2. The command exits with a status 0, despite making edits (removing the double space).

Expected behavior The command should have exited with a status of 1, since edits were made.

Environment (please complete the following information):

• Version: 20.8b1
• OS and Python version: Arch Linux/Python 3.8.6