tests: define GIT_TEST_PROTOCOL_VERSION

tests: define GIT_TEST_PROTOCOL_VERSION

Define a GIT_TEST_PROTOCOL_VERSION environment variable meant to be used
from tests. When set, this ensures protocol.version is at least the
given value, allowing the entire test suite to be run as if this
configuration is in place for all repositories.

As of this patch, all tests pass whether GIT_TEST_PROTOCOL_VERSION is
unset or set to 0. Some tests fail when GIT_TEST_PROTOCOL_VERSION is set
to 1 or 2, but this will be dealt with in subsequent patches.

This is based on work by Ævar Arnfjörð Bjarmason.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>