t3701: adjust difffilter test

t3701: adjust difffilter test

In 42f7d45428e (add--interactive: detect bogus diffFilter output,
2018-03-03), we added a test case that verifies that the diffFilter
feature complains appropriately when the output is too short.

In preparation for the upcoming change where the built-in `add -p` is
taught to respect that setting, let's adjust that test a little. The
problem is that `echo too-short` is configured as diffFilter, and it
does not read the `stdin`. When calling it through `pipe_command()`, it
is therefore possible that we try to feed the `diff` to it while it is
no longer listening, and we receive a `SIGPIPE`.

The Perl code apparently handles this in a way similar to an
end-of-file, but taking a step back, we realize that a diffFilter that
does not even _look_ at its standard input is very unrealistic. The
entire point of this feature is to transform the diff, not to ignore it
altogether.

So let's modify the test case to reflect that insight: instead of
printing some bogus text, let's use a diffFilter that deletes the first
line of the diff instead.

This still tests for the same thing, but it does not confuse the
built-in `add -p` with that `SIGPIPE`.

Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>