Fix a bug introduced in #195 - the changes in that PR broke ipbb add if neither branch nor revision were specified (i.e. cloning the default branch).
Also improve the cloning behaviour with the -r revision so that if the --depth flag is used then exactly the specified number of commits are downloaded (rather than up to twice that number).
ipbb addif neither branch nor revision were specified (i.e. cloning the default branch).-rrevision so that if the--depthflag is used then exactly the specified number of commits are downloaded (rather than up to twice that number).