contrib: add credential helper for GnomeKeyring

contrib: add credential helper for GnomeKeyring

With this installed in your $PATH, you can store
git-over-http passwords in your keyring by doing:

git config credential.helper gnome-keyring

The code is based in large part on the work of John Szakmeister
who wrote the helper originally for the initial, unpublished
version of the credential helper protocol.

This version will pass t0303 if you do:

  GIT_TEST_CREDENTIAL_HELPER=gnome-keyring \
  ./t0303-credential-external.sh

Signed-off-by: Philipp A. Hartmann <pah@qo.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>