FR: implement option to start sub branch

[KES777]

It will be useful to have an option to start/stop branches from the current one (not develop).
When I start main feature branch this option will allow to divide its implementation by sub branches

[petervanderdoes]

This is implemented in my fork git-flow (AVH Edition)
The last commit in the repository was on Sep 25, 2012, the last release was on on Feb 14, 2011, it seems this version of git-flow has been abandoned.

Debian and Ubuntu both, have switched to my fork for the package git-flow. If you installed git-flow through their system you are most likely to be using my version already.

Checkout the changelog for more information about bugfixes and new features implemented in my fork.