When validating svn:eol-style properties, do the quick "is binary"

When validating svn:eol-style properties, do the quick "is binary"
check before the slower newline consistency check, just like 1.4 used
to.

To implement this, we make the svn_wc_canonicalize_svn_prop_get_file_t
accept NULL values for mime_type and stream, and call it twice from
the validator.

* subversion/include/svn_wc.h
  (svn_wc_canonicalize_svn_prop_get_file_t): Change documentation to
   allow both output arguments to be null.

* subversion/libsvn_client/prop_commands.c
  (get_file_for_validation): Only fetch MIME type if mime_type is
   non-NULL.  (No change needed for stream, because svn_ra_get_file
   already has the proper semantics.)

* subversion/libsvn_wc/props.c
  (get_file_for_validation): Only fetch MIME type if requested; only
   fetch fetch file if requested.
  (validate_eol_prop_against_file): Call getter twice; check just the
   MIME type the first time, and just the contents the second time.

git-svn-id: http://svn.collab.net/repos/svn/trunk@28721 612f8ebc-c883-4be0-9ee0-a4e9ef946e3a