tests: introduce test for rebasing on named branches with closed heads

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.

rebase: improve error message for empty --rev set Before, it just said 'nothing to rebase' in this case. Now, it aborts mentioning the reason: 'empty "rev" 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.

rebase: test for empty dest revision

help: branch names primarily denote the tipmost unclosed branch head Was the behavior correct and the description wrong so it should be updated as in this patch? Or should the code work as the documentation says? Both ways could make some sense ... but none of them are obvious in all cases. One place where it currently cause problems is when the current revision has another branch head that is closer to tip but closed. 'hg rebase' refuses to rebase to that as it only see the tip-most unclosed branch head which is the current revision. /me kind of likes named branches, but no so much how branch closing works ...