idl fetch/update a single thrift file

[Willyham]

It would be nice to be able to update or fetch a single thrift file. Whilst I think that updating everything is a great default, there are often times when updating a single file is more practical. Working on a project in which many people are adding and updating thrift files results in many painful conflicts.

[jacobgreenleaf]

Documentation on what would happen if you manually patched the idl/meta.json would be useful as well.

[kriskowal]

Ensuring that idl fetch and update write out a consistent snapshot of the registry is an explicit goal. I think it’s sufficient to work around this limitation by using git checkout to restore unwanted changes after an update, but would love to hear your feedback.