When the ident attributes is set, get_stream_filter() did not obey
core.autocrlf=true, and the file was checked out with LF.
Change the rule when a streaming filter can be used:
- if an external filter is specified, don't use a stream filter.
- if the worktree eol is CRLF and "auto" is active, don't use a stream filter.
- Otherwise the stream filter can be used.
Add test cases in t0027.
Signed-off-by: Torsten Bögershausen <tboegi@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>