When the visited TAGS file was removed, commands that depend on TAGS
such as auto-completion may invoke an interactive question whether a
user wants to re-read the file.  From that point on, the question will
be asked over and over, because the file no longer exists, which
results in mtime mismatch and inability to "fix the mismatch" by
reading from the file.  Fix that by simply ignoring the mismatch if
the file no longer exists.