Don't barf when git fails

I'm not totally sold this is the best idea, but I felt like it while
working

bin/overview-by-date

@@ -64,7 +64,11 @@ def _git_checkout_by_date(repo, timepoint):
         'checkout',
         commit,
     ]
-    output = subprocess.check_output(command, stderr=subprocess.STDOUT)
+    try:
+        output = subprocess.check_output(command, stderr=subprocess.STDOUT)
+    except subprocess.CalledProcessError:
+        LOGGER.warning("Failed to execute '%s' in %s", command, abspath)
+        return ''
     LOGGER.debug("Checked out %s at %s", repo, timepoint.date().isoformat())
     return output