Force diff output to color gitlab-housekeeper preview

What does this MR do and why?

Force diff output to color gitlab-housekeeper preview

The diff output does not use color when shelled out from Ruby by default. Setting --color=always seems to fix this. Since this is only used to send directly to puts straight after this also seems safe. If we were using this diff content for something else it might not be great but the scope of this line of code is pretty clearly only for the preview output.

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