entry: fix filter lookup

When looking up the stream filter, write_entry() should be passing the
path of the file in the repository, not the path to which the content is
going to be written.  This allows the file to be correctly looked up
against the .gitattributes files in the working tree.

This change makes the streaming case match the non-streaming case which
passes ce->name to convert_to_working_tree later in the same function.

The two tests added here test the different paths through write_entry
since the CRLF filter is a streaming filter but the user-defined smudge
filter is not streamed.

Signed-off-by: John Keeping <john@keeping.me.uk>