t: add test harness for external credential helpers

We already have tests for the internal helpers, but it's
nice to give authors of external tools an easy way to
sanity-check their helpers.

If you have written the "git-credential-foo" helper, you can
do so with:

  GIT_TEST_CREDENTIAL_HELPER=foo \
  make t0303-credential-external.sh

This assumes that your helper is capable of both storing and
retrieving credentials (some helpers may be read-only, and
they will fail these tests).

If your helper supports time-based expiration with a
configurable timeout, you can test that feature like this:

  GIT_TEST_CREDENTIAL_HELPER_TIMEOUT="foo --timeout=1" \
  make t0303-credential-external.sh

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0303-credential-external.sh

@@ -0,0 +1,39 @@
+#!/bin/sh
+
+test_description='external credential helper tests'
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-credential.sh
+
+pre_test() {
+	test -z "$GIT_TEST_CREDENTIAL_HELPER_SETUP" ||
+	eval "$GIT_TEST_CREDENTIAL_HELPER_SETUP"
+
+	# clean before the test in case there is cruft left
+	# over from a previous run that would impact results
+	helper_test_clean "$GIT_TEST_CREDENTIAL_HELPER"
+}
+
+post_test() {
+	# clean afterwards so that we are good citizens
+	# and don't leave cruft in the helper's storage, which
+	# might be long-term system storage
+	helper_test_clean "$GIT_TEST_CREDENTIAL_HELPER"
+}
+
+if test -z "$GIT_TEST_CREDENTIAL_HELPER"; then
+	say "# skipping external helper tests (set GIT_TEST_CREDENTIAL_HELPER)"
+else
+	pre_test
+	helper_test "$GIT_TEST_CREDENTIAL_HELPER"
+	post_test
+fi
+
+if test -z "$GIT_TEST_CREDENTIAL_HELPER_TIMEOUT"; then
+	say "# skipping external helper timeout tests"
+else
+	pre_test
+	helper_test_timeout "$GIT_TEST_CREDENTIAL_HELPER_TIMEOUT"
+	post_test
+fi
+
+test_done