t5512.40 sometimes dies by SIGPIPE

The last test in t5512 we recently added seems to be flaky. Running $ make && cd t && sh ./t5512-ls-remote.sh --stress shows that "git ls-remote foo::bar" exited with status 141, which means we got a SIGPIPE. This test piece was introduced by 9e89dcb6 (builtin/ls-remote: fall back to SHA1 outside of a repo, 2024-08-02) and is pretty much independent from all other tests in the script (it can even run standalone with everything before it removed). The transport-helper.c:get_helper() function tries to write to the helper. As we can see the helper script is very short and can exit even before it reads anything, when get_helper() tries to give the first command, "capabilities", the helper may already be gone. A trivial fix, presented here, is to make sure that the helper reads the first command it is given, as what it writes later is a response to that command. I however would wonder if the interactions with the helper initiated by get_helper() should be done on a non-blocking I/O (we do check the return value from our write(2) system calls, do we?). Signed-off-by: Junio C Hamano <gitster@pobox.com>