According to the git-check-attr synopsis, if the '--stdin' option is
used then no pathnames are expected on the command line. Change the
behavior to match this description; namely, if '--stdin' is used but
not '--', then treat all command-line arguments as attribute names.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>