Fix doc build if git is absent

When building packages if git is absent, then we should not set
html_last_updated_fmt. It can still be set via the -D switch
when building with sphinx-build.

Change-Id: Icfeb9354fff36776f5c83d3da3ab39ce2bb1dfb3
Closes-Bug: #1552251

doc/source/conf.py

@@ -14,6 +14,7 @@
 import sys
 import os
 import subprocess
+import warnings
 
 # Build the plugin registry
 def build_plugin_registry(app):
@@ -140,9 +141,13 @@ html_static_path = ['_static']
 # using the given strftime format.
 git_cmd = ["git", "log", "--pretty=format:'%ad, commit %h'", "--date=local",
    "-n1"]
-html_last_updated_fmt = subprocess.Popen(git_cmd,
-                                         stdout=subprocess.PIPE).\
-                                         communicate()[0]
+try:
+    html_last_updated_fmt = subprocess.Popen(git_cmd,
+                                             stdout=subprocess.PIPE).\
+                                             communicate()[0]
+except Exception:
+    warnings.warn('Cannot get last updated time from git repository. '
+                  'Not setting "html_last_updated_fmt".')
 
 # If true, SmartyPants will be used to convert quotes and dashes to
 # typographically correct entities.