Fix bogus '. missing from VCS' error

I've only started seeing this error today, only on Appveyor (but not on
my Windows Jenkins instance), and only on Python 2.7.

Somehow the sdist archive mentions the top-level directory name twice,
once with a slash, and once without.  Possibly my own preprocessors
could've been adding the second entry.  Anyway, strip it.

Again, I've no idea how I never saw this before.  Did Appveyor update
their Python or something?  Maybe a newer setuptools caused this?

appveyor.yml

@@ -19,6 +19,9 @@ install:
   - choco install bzr
   - "set PATH=C:\\Program Files (x86)\\Bazaar;%PATH%"
   - bzr --version
+  - git --version
+  - svn --version
+  - hg --version
 
 build: off
 

check_manifest.py

@@ -283,6 +283,9 @@ def strip_toplevel_name(filelist):
         >>> strip_toplevel_name(['a', 'a/b', 'a/c', 'a/c/d'])
         ['b', 'c', 'c/d']
 
+        >>> strip_toplevel_name(['a', 'a/', 'a/b', 'a/c', 'a/c/d'])
+        ['b', 'c', 'c/d']
+
         >>> strip_toplevel_name(['a/b', 'a/c', 'a/c/d'])
         ['b', 'c', 'c/d']
 
@@ -300,7 +303,7 @@ def strip_toplevel_name(filelist):
         if not name.startswith(prefix):
             raise Failure("File doesn't have the common prefix (%s): %s"
                           % (name, prefix))
-    return [name[len(prefix):] for name in names]
+    return [name[len(prefix):] for name in names if name != prefix]
 
 
 def add_prefix_to_each(prefix, filelist):

tests.py

@@ -1394,19 +1394,20 @@ def test_forgot_to_git_add_anything(self):
     def test_all_is_well(self):
         from check_manifest import check_manifest
         self._create_repo_with_code()
-        self.assertTrue(check_manifest())
+        self.assertTrue(check_manifest(), sys.stderr.getvalue())
 
     def test_relative_pathname(self):
         from check_manifest import check_manifest
         subdir = self._create_repo_with_code_in_subdir()
-        self.assertTrue(check_manifest(subdir))
+        self.assertTrue(check_manifest(subdir), sys.stderr.getvalue())
 
     def test_relative_python(self):
         # https://github.com/mgedmin/check-manifest/issues/36
         from check_manifest import check_manifest
         subdir = self._create_repo_with_code_in_subdir()
         python = os.path.relpath(sys.executable)
-        self.assertTrue(check_manifest(subdir, python=python))
+        self.assertTrue(check_manifest(subdir, python=python),
+                        sys.stderr.getvalue())
 
     def test_python_from_path(self):
         # https://github.com/mgedmin/check-manifest/issues/57
@@ -1418,7 +1419,8 @@ def test_python_from_path(self):
                 if shutil.which(python):
                     break
         subdir = self._create_repo_with_code_in_subdir()
-        self.assertTrue(check_manifest(subdir, python=python))
+        self.assertTrue(check_manifest(subdir, python=python),
+                        sys.stderr.getvalue())
 
     def test_suggestions(self):
         from check_manifest import check_manifest