rebase: remove old code for handling empty rebaseset

rebase: improve error message for --base being empty or causing emptiness Before it just said 'nothing to rebase'. Now 'if "base" is an empty set: abort: empty "base" revision set - can't compute rebase set If the set of changesets to rebase can't be found from "base", it will fail as before but with more explanation of what the problem was. The name of the "base" option is not obvious - it is more like "samples identifying the branch to rebase". The error messages for problems with the specified "base" value will use that term and might thus also not be obvious, but at least they are consistent with the option name. The name "base" will not be used if the base only was specified implicitly as the working directory parent.

rebase: improve error message for empty --source set Before, it just said 'nothing to rebase' in this case. Now, it aborts mentioning the reason: 'empty "source" revision set'. Specifying revisions that cannot be rebased is a 'soft' error, but specifying an empty set deserves an abort that explains exactly what the problem is.