revset: avoid a ValueError when 'only()' is given an empty set

This previously died in _revdescendants() taking the min() of the first set to
only(), when it was empty.  An empty second set already worked.  Likewise,
descendants() already handled an empty set.

mercurial/revset.py

@@ -399,6 +399,9 b' def only(repo, subset, x):'
     args = getargs(x, 1, 2, _('only takes one or two arguments'))
     include = getset(repo, spanset(repo), args[0]).set()
     if len(args) == 1:
+        if len(include) == 0:
+            return baseset([])
+
         descendants = set(_revdescendants(repo, include, False))
         exclude = [rev for rev in cl.headrevs()
             if not rev in descendants and not rev in include]

tests/test-revset.t

@@ -427,6 +427,16 b' min: empty on unordered set'
   7
   8
   9
+
+Test empty set input
+  $ log 'only(p2())'
+  $ log 'only(p1(), p2())'
+  0
+  1
+  2
+  4
+  8
+  9
   $ log 'outgoing()'
   8
   9