Add twgit release update

[vchabot]

Hello,

I thought to a feature which could be useful in some cases:
When you have x features in your release, and you need to merge them in the release, instead of typing twgit feature merge-into-release XXX for each feature, I think a command like twgit release update could find every features which need to be updated, and then update then into the release.

What do you think about this command and its utility?

[geoffroy-aubry]

Not for every day, but can be useful.
Code should be easy reusing code of twgit release list and twgit feature merge-into-release.