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>

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