narrowcommands: drop unnecessary adding of '' for root directory

It's now included by util.dirs().

Differential Revision: https://phab.mercurial-scm.org/D6405

hgext/narrow/narrowcommands.py

@@ -216,7 +216,7 b' def _narrow(ui, repo, remote, commoninc,'
                     todelete.append(f)
             elif f.startswith('meta/'):
                 dir = f[5:-13]
-                dirs = [''] + sorted(util.dirs({dir})) + [dir]
+                dirs = sorted(util.dirs({dir})) + [dir]
                 include = True
                 for d in dirs:
                     visit = newmatch.visitdir(d)