t/helper: ignore only executable files

t/helper: ignore only executable files

This patch conceptually reverts 44103f4197 (t/helper: ignore
everything but sources, 2017-12-12).  Back in those days we did have a
lot of separate test helper executables under 't/helper', and its
'.gitignore' did get out of sync every once in a while.

Since then, however, most of those separate executables were
integrated into a single 'test-tool' command [1], and new test helpers
are added as new subcommands, so the chances of that '.gitignore'
getting out of sync again are much lower.  And even if a contributor
were not careful enough and submits a patch that adds a new executable
under 't/helper' but forgets to update '.gitignore' accordingly, our
CI builds would catch it in a timely manner [2].

Ignoring everything but sources has the drawback that building an
older version of Git (e.g. during bisecting) creates all those
executables, and after going back to e.g. current 'master' the usual
cleanup commands like 'make clean' or 'git clean -fd' don't remove
them (the former doesn't know about them, and the latter doesn't
remove ignored files).

So let's ignore only the executable files under 't/helper/, i.e.
'test-tool' and the three other remaining executables that could not
be integrated into 'test-tool' (no need to ignore object files, as
they are already ignored by our toplevel '.gitignore').

[1] The topic starting with efd71f8913 (t/helper: add an empty
    test-tool program, 2018-03-24), and leading up to the merge commit
    27f25845cf (Merge branch 'nd/combined-test-helper', 2018-04-11).

[2] b92cb86ea1 (travis-ci: check that all build artifacts are
    .gitignore-d, 2017-12-31)

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>