Use default flags only for new-* path

[alexbiehl]

Fixes #4581.
With 1e90ae4 flags from the global
configuration would be ignored. This patch makes sure the defaults are
mixed in only for the new-build code path.

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[mention-bot]

@alexbiehl, thanks for your PR! By analyzing the history of the files in this pull request, we identified @23Skidoo, @ezyang and @dcoutts to be potential reviewers.

[23Skidoo]

Ah yeah, that's because command line flags are mappended on top of the ones loaded from the config, so when they're non-mempty from the beginning, they always override whatever was in the config. I think we should just revert 1e90ae4.

[23Skidoo]

I'd be more happy if instead of doing this we applied the defaults directly inside Cmd*.*Action. That'd be a tad more verbose, but also more explicit.

So e.g. for CmdBuild.buildAction you'd have:

buildAction cliFlags@(_configFlags, _configExFlags, _installFlags, _haddockFlags) ... =
let (configFlags, configExFlags, installFlags, haddockFlags) = applyFlagDefaults cliFlags
...

[hvr]

@23Skidoo we could use ViewPatterns, then it would look like

buildAction (applyFlagDefaults -> (configFlags, configExFlags, installFlags, haddockFlags))

EDIT: syntax typo

[hvr]

@23Skidoo if we only/just revert 1e90ae4 then new-build regresses again by running into errors whenever fromFlag is called on some NoFlag. There was a reason for 1e90ae4...

[hvr]

@23Skidoo it's all green! :-)