Handle errors from git describe

The git describe command reports errors if there are no tags to be used
to get a description. Trap the errors in places where it is reasonable
to encounter that situation.

Change-Id: I436af248b1740c493b14178cbbe59c8b0474aa94

openstack_releases/cmds/list_changes.py

@@ -161,9 +161,12 @@ def main():
             # Show more details about the commit being tagged.
             print()
             print('git describe %s' % project['hash'])
-            subprocess.call(
-                ['git', 'describe', project['hash']],
-                cwd=os.path.join(workdir, project['repo']),
-            )
+            try:
+                subprocess.check_call(
+                    ['git', 'describe', project['hash']],
+                    cwd=os.path.join(workdir, project['repo']),
+                )
+            except subprocess.CalledProcessError as e:
+                print('WARNING: Could not run git describe: %s' % e)
 
     return 0

openstack_releases/gitutils.py

@@ -90,7 +90,10 @@ def check_ancestry(workdir, repo, old_version, sha):
 
 
 def get_latest_tag(workdir, repo):
-    return subprocess.check_output(
-        ['git', 'describe', '--abbrev=0'],
-        cwd=os.path.join(workdir, repo),
-    ).strip()
+    try:
+        return subprocess.check_output(
+            ['git', 'describe', '--abbrev=0'],
+            cwd=os.path.join(workdir, repo),
+        ).strip()
+    except subprocess.CalledProcessError:
+        return None