revert: add a fullreposet class

Every revset evaluation starts from `subset = spanset(repo)` and a lot of
revset predicates build a `spansetrepo` for their internal needs.

`spanset` is a generic class that can handle any situation. As a result a lot
of operation between spanset result in an `orderedlazyset`, a safe object but
suboptimal in may situation.

So we introduce a `fullreposet` class where some of the operation will be
overwritten to produce more interesting results.

mercurial/revset.py

@@ -2851,5 +2851,14 b' class spanset(_orderedsetmixin):'
     def filter(self, l):
         return orderedlazyset(self, l, ascending=self.isascending())
 
+class fullreposet(spanset):
+    """a set containing all revisions in the repo
+
+    This class exists to host special optimisation.
+    """
+
+    def __init__(self, repo):
+        super(fullreposet, self).__init__(repo)
+
 # tell hggettext to extract docstrings from these functions:
 i18nfunctions = symbols.values()