Fix doc build if git is absent

When building packages if git is absent, do not try to get the git sha
via the git cli. That can not work.

Change-Id: I3bde4df045287cbda07c9447ef3397de74f42910
Closes-Bug: #1677463

doc/source/conf.py

@@ -13,6 +13,7 @@
 
 import os
 import sys
+import warnings
 
 import openstackdocstheme
 
@@ -67,7 +68,12 @@ release = '1.0'
 # These variables are passed to the logabug code via html_context.
 giturl = u'http://git.openstack.org/cgit/openstack/python-openstacksdk/tree/doc/source'
 git_cmd = "/usr/bin/git log | head -n1 | cut -f2 -d' '"
-gitsha = os.popen(git_cmd).read().strip('\n')
+try:
+    gitsha = os.popen(git_cmd).read().strip('\n')
+except Exception:
+    warnings.warn("Can not get git sha.")
+    gitsha = "unknown"
+
 bug_tag = "docs"
 # source tree
 pwd = os.getcwd()