blame,cat-file: Prepare --textconv tests for correctly-failing conversion program

blame,cat-file: Prepare --textconv tests for correctly-failing conversion program

The textconv filter is sometimes incorrectly ran on a temporary file
whose content is the target of a symbolic link, instead of actual file
content. Prepare to test this by marking the content of the file to
convert with "bin:", and let the helper die if "bin:" is not found in
the file content.

NOTE: I've changed $@ to $1 in helper becase textconv program "should
take a single argument" (see Documentation/gitattributes.txt), so
making this more explicit makes sense and also helps to avoid
problems with feeding arguments to echo.

Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>
Reviewed-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>