If I sign a file (or an email, or anything else), I get the following
message as of GnuPG 2.1.10:

  vauxhall ok % gpg -b input-2.txt
  gpg: using "1F52F68B!" as default secret key

This is annoying because it causes mutt to pause so it can render the
output from GnuPG every time I send a mail.  I'm fully aware that GnuPG
is using the secret key I specified in my config file and would rather
not be told about it.

This will also affect everyone who uses only a subkey and therefore will
generally specify the default key explicitly.