Feature request: Rebase in progress indicator

[mgild]

On more than one occasion I mess up my tree because I forget a rebase is in progress. A "Rebase in progress" indicator would be extremely helpful in git_super_status.

[starcraftman]

@mgild This repo isn't very active, I suggest you try my fork out. I've just merged support for a rebase indicator. At present, it only works with the python interface. You can see a summary of changes in #123 .