revset: use `subset &` in `divergent`

This takes advantage of the `fullreposet` smartness.

revset  #0: divergent()
0) wall 0.002047 comb 0.000000 user 0.000000 sys 0.000000 (best of 813)
1) wall 0.000052 comb 0.000000 user 0.000000 sys 0.000000 (best of 22757)

mercurial/revset.py

@@ -734,7 +734,7 b' def divergent(repo, subset, x):'
     # i18n: "divergent" is a keyword
     getargs(x, 0, 0, _("divergent takes no arguments"))
     divergent = obsmod.getrevs(repo, 'divergent')
-    return subset.filter(divergent.__contains__)
+    return subset & divergent
 
 def draft(repo, subset, x):
     """``draft()``