CLI design for diffing commands

[merijn]

I'm finally polishing up the config diffing code I wrote in December, which would simplify comparing one generated CI config with another. The CLI UX for this is a bit tricky, though. There's a number of comparisons that make sense:

• Comparing a config file against the default config
• Comparing a Travis/GitHub/bash file against the default config
• Comparing 2 config files
• Comparing 2 CI files against each other
• Comparing a CI file against a config file

The "obvious" UX is diff with 1 argument compares against the default config and with 2 arguments it diffs those two files. There are two tricky UX parts:

• What would it mean if there are commandline options passed to the diff command
• How do we know if a file is a config file?

The first issue can be trivially solved by refactoring some of the optparse code in HaskellCI.Cli to move the flags/options into the subcommands and simply disallowing commandline options to a diff subcommand. Becomes somewhat more challenging with the future goal of eliminating subcommands.

The second issue is trickier. CI files are easy to detect since they always use REGENDATA, which already has extraction code. But how to detect whether something is a config file? Do we simply assume a lack of REGENDATA means something is a haskell-ci config file?