Add: git rebase-branches command

**This is a proposal!**

This patch adds a command for automatic rebase of several branches by
branch list or by a pattern for finding these branches using (e)grep.

This is still work in progress and **not tested yet**. Wanted to hear what you guys think about this!

Motivation: I use something similar (always live-hacked in my bash) almost every day, when rebasing N patch-branches in my kernel tree.