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?).

Backported-from: e1e0d305c41 (t5512.40 sometimes dies by SIGPIPE, 2024-09-13)
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t5512-ls-remote.sh

@@ -405,6 +405,7 @@ test_expect_success 'v0 clients can handle multiple symrefs' '
 test_expect_success 'helper with refspec capability fails gracefully' '
 	mkdir test-bin &&
 	write_script test-bin/git-remote-foo <<-EOF &&
+	read capabilities
 	echo import
 	echo refspec ${SQ}*:*${SQ}
 	EOF