Merge: warn on wrong feature/hotfix command

Now you'll have to confirm if you want to hotfix-merge a feature, or
feature-merge a hotfix.  This should reduces silly mistakes.

`current_hotfix_branch` no longer exits if we're not on a hotfix.  It was
strange that we did that sort of checking for hotfixes, but not features, so
now we should implement that sort of constraint, where appropriate, in the
calling functions.  This is sorta reverting part of 5ee8064d.

Inspired by #114.
Finish: exit on wrong feature/hotfix command

Don't allow users to request a hotfix branch get merged into the development
branch, or a feature branch into the stable branch, because it screws things up
and really isn't what they want.

Fixes #114.
hotfix.rb: s/feature/hotfix/ in branch name checks
Merge finish--warn-on-wrong-one (#115) into master

Check branches for appropriate feature/hotfix command

It's fairly commmon to accidentally use hotfix-finish on a feature, or
that sort of thing.  Let's try to stem those errors.

Fixes #114.