merge: check for untracked files more precisely (issue3400)

This fixes the regression, but still leaves the long-standing issue
that merge doesn't cope with trying to merge files and directories.

mercurial/merge.py

@@ -83,7 +83,7 b' class mergestate(object):'
 
 def _checkunknownfile(repo, wctx, mctx, f):
     return (not repo.dirstate._ignore(f)
-        and os.path.exists(repo.wjoin(f))
+        and os.path.isfile(repo.wjoin(f))
         and repo.dirstate.normalize(f) not in repo.dirstate
         and mctx[f].cmp(wctx[f]))