Handle git submodules in bundlebuilder

Git submodules are only listed by their root directory in the
output of "git ls-files".  Therefore, we must handle this case
so that the files from the module are included in the install
and dist_xo commands.

To reproduce the issue, you could do something like the following:

    cd pippy
    git clone https://github.com/samdroid-apps/collabwrapper
    git add collabwrapper
    python setup.py install  # or just osbuild run

src/sugar3/activity/bundlebuilder.py

@@ -168,12 +168,15 @@ class Packager(object):
         if not os.path.exists(self.config.dist_dir):
             os.mkdir(self.config.dist_dir)
 
-    def get_files_in_git(self):
+    def get_files_in_git(self, root=None):
+        if root is None:
+            root = self.config.source_dir
+
         git_ls = None
         try:
             git_ls = subprocess.Popen(['git', 'ls-files'],
                                       stdout=subprocess.PIPE,
-                                      cwd=self.config.source_dir)
+                                      cwd=root)
         except OSError:
             logging.warn('Packager: git is not installed, '
                          'fall back to filtered list')
@@ -196,6 +199,13 @@ class Packager(object):
                             ignore = True
                             break
                     if not ignore:
+                        sub_path = os.path.join(root, line)
+                        if os.path.isdir(sub_path) \
+                           and os.path.isdir(os.path.join(sub_path, '.git')):
+                            sub_list = self.get_files_in_git(sub_path)
+                            for f in sub_list:
+                                files.append(os.path.join(line, f))
+                        else:
                             files.append(line)
 
                 for pattern in IGNORE_FILES: