Allow tracking empty directories

Handle creating and removing placeholders files for versioning empty
directories as suggested in [1].

[1]: https://git.wiki.kernel.org/index.php/GitFaq#Can_I_add_empty_directories.3F

Author: embs

gitless/cli/file_cmd.py

@@ -35,11 +35,16 @@ def f(args, repo):
 
     for fp in args.files:
       try:
+        empty_dir = fp.endswith('.glkeep')
         getattr(curr_b, subcmd + '_file')(fp)
+        if empty_dir:
+          fp = fp.replace('.glkeep', '')
         pprint.ok(
-            'File {0} is now a{1} {2}{3}d file'.format(
+            '{0} {1} is now a{2} {3}{4}d {5}'.format(
+              'Empty directory' if empty_dir else 'File',
               fp, 'n' if subcmd.startswith(VOWELS) else '', subcmd,
-              '' if subcmd.endswith('e') else 'e'))
+              '' if subcmd.endswith('e') else 'e',
+              'directory' if empty_dir else 'file'))
       except KeyError:
         pprint.err('Can\'t {0} non-existent file {1}'.format(subcmd, fp))
         success = False

gitless/cli/helpers.py

@@ -129,6 +129,9 @@ def process_paths():
                 self.skip_dir_cb(curr_dir_rel)
               dirs[:] = []
               continue
+            if not fps:
+              open(curr_dir + "/.glkeep.", "a").close()
+              fps.append(".glkeep")
             for fp in fps:
               yield os.path.join(curr_dir_rel, fp)
         else:

gitless/tests/test_e2e.py

@@ -63,6 +63,18 @@ def assert_not_in_repo(*cmds):
       gl.status, gl.diff, gl.commit, gl.branch, gl.merge, gl.fuse, gl.remote,
       gl.publish, gl.history)
 
+class TestTrackEmptyDir(TestEndToEnd):
+
+  def test_tracked_empty_dir(self):
+    dir_to_track = 'wanted_empty_dir'
+    dir_to_track_path = os.path.join(self.path, dir_to_track)
+    os.mkdir(dir_to_track_path)
+    expected_out = 'Empty directory {0}/ is now a tracked directory'.format(
+      dir_to_track)
+
+    out = utils.stdout(gl.track(dir_to_track_path))
+
+    self.assertIn(expected_out, out, 'Empty dir wasn\'t tracked')
 
 class TestBasic(TestEndToEnd):