Improve error handling
Use rev-parse on the command line arguments

Now the user may run 'greb @' or 'greb foo@{u}', i.e.
Checkout original branch on interrupt signal
Improve help output
Add git option greb.local
Change git option for the color of the commands

It's backwards incompatible.
It's color.greb.command now.
Highlight current branch
Highlight remote branches