Add new feature subcommand called "clone/update" about gitflow

[Zoramite]

What happens when it isn't a feature? What if it was a code review for a hotfix, etc? Or if the remote repository uses a different prefix for their development?

[nvie]

Well, the subcommand could analogously be added to the other commands, too. It is indeed quite useful to be able to share release/hotfix branches among developers.

I think we can safely assume that the remote is using the same prefix for feature branches. Providing the flexibility to use different remote naming styles does not outweigh the complexity it requires in my opinion.

[Zoramite]

That makes sense. I was thinking that you could use a --prefix featurette style when you are tracking/cloning the feature.

I'm a bit ignorant of what it would take to accomplish the task but it seems you could store the prefix information with the rest of the tracking information for the branch.

[nvie]

Maybe "pull" would be a better name for the feature?

[nvie]

Initial implementation in f68d405. See commit message for details and example usage.