Fix sorting of git versions

If you run "git tag", the versions are listed in alphabetical order and
thus 1.9 is the last one - and not 1.12. Sort properly.

Change-Id: I64b6835b951d7d3547415743505d7a2e31dac632
Closes-Bug: #1702143

openstackdocstheme/__init__.py

@@ -24,7 +24,7 @@ def _get_other_versions(app):
     if not app.config.html_theme_options.get('show_other_versions', False):
         return []
 
-    git_cmd = ["git", "tag", "--merged"]
+    git_cmd = ["git", "tag", "--sort=v:refname", "--merged"]
     try:
         raw_version_list = subprocess.Popen(
             git_cmd, stdout=subprocess.PIPE).communicate()[0]