By default, Credo attributes input from STDIN as though it came from a
file named `stdin`. This change passes the buffer's filename, too, so
that Credo can use that information when applying its configuration.
This is a nice improvement because files like `mix.exs` are normally
excluded from Credo-based linting. Previously, ALE would show lint
warnings for those files as they were edited. Now, they are correctly
honor the Credo configuration and don't produce lint output.
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