Convert prompt answers to lower case before checking

When prompting for yes/no answers, convert the answer to lower
case before comparing.  This makes it easier to catch answers
like "Yes", "yes", and "YES" with a comparison only for "yes".

Change-Id: I06da8281cec81a7438ebb46ddaf3344d12abe1eb
diff --git a/subcmds/upload.py b/subcmds/upload.py
index 39721ac..925652c 100644
--- a/subcmds/upload.py
+++ b/subcmds/upload.py
@@ -186,8 +186,8 @@
         print '         %s' % commit
 
       sys.stdout.write('to %s (y/N)? ' % remote.review)
-      answer = sys.stdin.readline().strip()
-      answer = answer in ('y', 'Y', 'yes', '1', 'true', 't')
+      answer = sys.stdin.readline().strip().lower()
+      answer = answer in ('y', 'yes', '1', 'true', 't')
 
     if answer:
       if len(branch.commits) > UNUSUAL_COMMIT_THRESHOLD: