baseset: stop inheriting from built-in list class The baseset is doing more and more smartset magic and using its list-like property less and less. So we store the list of revisions in an explicit attribute and stop inheriting. This requires reimplementing some basic methods.

strip: stop calling `remove` on smartset The `remove` method is not part of the smartset specification. We use a plain old list comprehension instead.

rebase: transform the smartset to a list before comparing with a list This is highly suboptimal but smartsets are not comparable to lists yet.

merge.update: use `first` instead of direct indexing This makes it compatible with all smartset classes.

qimport: use `first` and `last` instead of direct indexing This makes it compatible with all smartset classes.

rebase: use `last` instead of direct indexing This makes it compatible with all smartset classes.