Require 'CHROME_HEADLESS' to use 'git-number'.

git_number.py

Index: git_number.py
diff --git a/git_number.py b/git_number.py
index 7fa0398af775abfafa33caa1e803ffe508fcbf23..6997f51b3d0ffd6a21383949315ff7dbdcd6109d 100755
--- a/git_number.py
+++ b/git_number.py
@@ -252,6 +252,13 @@ def main():  # pragma: no cover
   levels = [logging.ERROR, logging.INFO, logging.DEBUG]
   logging.basicConfig(level=levels[min(opts.verbose, len(levels) - 1)])
 
+  # 'git number' should only be used on bots.
+  if os.getenv('CHROME_HEADLESS') != '1':
+    logging.error("'git-number' is an infrastructure tool that is only "
+                  "intended to be used internally by bots. Developers should "
+                  "use the 'Cr-Commit-Position' value in the commit's message.")
+    return 1
+
   try:
     if opts.reset:
       clear_caches(on_disk=True)