apollo client:codegen --mergeInFieldsFromFragmentSpreads option does nothing

`mergeInFieldsFromFragmentSpreads` is on by default. The command line option allows you to only turn it on. There is no way turn it off.

`mergeInFieldsFromFragmentSpreads` is implemented as an oclif boolean flag so alternatives like `apollo client:codegen --mergeInFieldsFromFragmentSpreads=false` don't work.

There is an older issue: https://github.com/apollographql/apollo-tooling/issues/537 where the meaning of the flag was inverted but this was not done everywhere.

Potentially the fix is as simple as changing this line: https://github.com/apollographql/apollo-tooling/blob/master/packages/apollo/src/commands/client/codegen.ts#L235
From `flags.mergeInFieldsFromFragmentSpreads` to `!!flags.mergeInFieldsFromFragmentSpreads`

It might also be necessary to update the docs.

Start in packages/apollo/src/commands/client/codegen.ts around line 235 and inspect the oclif flag definition and how its value is passed to code generation. Check the client codegen documentation for the option, then run the command with the option enabled and disabled to confirm that both states are respected and the documented behavior matches.

Written by the indexing model from the issue text.