NOT FOR MERGE gen-content.py takes a branch ref

Let's add a branch ref for external-sources (to temporarily enable
previews of fluxcd/community#396)

Signed-off-by: Kingdon Barrett <kingdon+github@tuesdaystudios.com>

hack/gen-content.py

@@ -76,9 +76,14 @@ def __del__(self):
         shutil.rmtree(self.temp_dir)
 
     def clone_repo(self):
-        subprocess.call([
-            'git', 'clone', '--depth=1', '-q',
-            self.gh_source, self.dest])
+        if self.gh_source == 'https://github.com/fluxcd/community':
+            subprocess.call([
+                'git', 'clone', '--depth=1', '-q',
+                self.gh_source, self.dest, '--branch', 'add-calendar-table'])
+        else:
+            subprocess.call([
+                'git', 'clone', '--depth=1', '-q',
+                self.gh_source, self.dest])
 
     def rewrite_links(self, out_file):
         '''