t0300: make "quit" helper more realistic

We test a toy credential helper that writes "quit=1" and confirms that
we stop running other helpers. However, that helper is unrealistic in
that it does not bother to read its stdin at all.

For now we don't send any input to it, because we feed git-credential a
blank credential. But that will change in the next patch, which will
cause this test to racily fail, as git-credential will get SIGPIPE
writing to the helper rather than exiting because it was asked to.

Let's make this one-off helper more like our other sample helpers, and
have it source the "dump" script. That will read stdin, fixing the
SIGPIPE problem. But it will also write what it sees to stderr. We can
make the test more robust by checking that output, which confirms that
we do run the quit helper, don't run any other helpers, and exit for the
reason we expected.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

t/t0300-credentials.sh

@@ -22,6 +22,11 @@ test_expect_success 'setup helper scripts' '
 	exit 0
 	EOF
 
+	write_script git-credential-quit <<-\EOF &&
+	. ./dump
+	echo quit=1
+	EOF
+
 	write_script git-credential-verbatim <<-\EOF &&
 	user=$1; shift
 	pass=$1; shift
@@ -291,11 +296,16 @@ test_expect_success 'http paths can be part of context' '
 
 test_expect_success 'helpers can abort the process' '
 	test_must_fail git \
-		-c credential.helper="!f() { echo quit=1; }; f" \
+		-c credential.helper=quit \
 		-c credential.helper="verbatim foo bar" \
-		credential fill >stdout &&
+		credential fill >stdout 2>stderr &&
 	>expect &&
-	test_cmp expect stdout
+	test_cmp expect stdout &&
+	cat >expect <<-\EOF &&
+	quit: get
+	fatal: credential helper '\''quit'\'' told us to quit
+	EOF
+	test_i18ncmp expect stderr
 '
 
 test_expect_success 'empty helper spec resets helper list' '