t0027: add tests for get_stream_filter()

When a filter is configured, a different code-path is used in convert.c
and entry.c via get_stream_filter(), but there are no test cases yet.

Add tests for the filter API by configuring the ident filter.
The result of the SHA1 conversion is not checked, this is already
done in other TC.

Add a parameter to checkout_files() in t0027.
While changing the signature, add another parameter for the eol= attribute.
This is currently unused, tests for e.g.
"* text=auto eol=lf" will be added in a separate commit.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>