revset: fix merge() to fall back to changectx API if wdir specified

I have a code which basically runs "0:wdir() & <user-revset>", and it crashed
if merge() were passed in.

mercurial/revset.py

@@ -1362,8 +1362,12 b' def merge(repo, subset, x):'
     getargs(x, 0, 0, _("merge takes no arguments"))
     cl = repo.changelog
     nullrev = node.nullrev
-    return subset.filter(lambda r: cl.parentrevs(r)[1] != nullrev,
-                         condrepr='<merge>')
+    def ismerge(r):
+        try:
+            return cl.parentrevs(r)[1] != nullrev
+        except error.WdirUnsupported:
+            return bool(repo[r].p2())
+    return subset.filter(ismerge, condrepr='<merge>')
 
 @predicate('branchpoint()', safe=True)
 def branchpoint(repo, subset, x):

tests/test-revset.t

@@ -2076,6 +2076,17 b" itself isn't returned unless it is expli"
   $ log 'parents(merge())'
   4
   5
+
+  $ hg merge 7
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ log '0:wdir() & merge()'
+  6
+  2147483647
+  $ hg update -qC .
+  $ log '0:wdir() & merge()'
+  6
+
   $ log 'p1(branchpoint())'
   0
   2