contrib: add credential helper for OS X Keychain

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

git config credential.helper osxkeychain

The code is based in large part on the work of Jay Soffian,
who wrote the helper originally for the initial, unpublished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=osxkeychain \
GIT_TEST_CREDENTIAL_HELPER_SETUP="export HOME=$HOME" \
./t0303-credential-external.sh

The "HOME" setup is unfortunately necessary. The test
scripts set HOME to the trash directory, but this causes the
keychain API to complain.

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