protocol test: let protocol.version override GIT_TEST_PROTOCOL_VERSION

protocol test: let protocol.version override GIT_TEST_PROTOCOL_VERSION

The GIT_TEST_PROTOCOL_VERSION environment variable can be used to
upgrade the version of Git protocol used in tests.  If both
GIT_TEST_PROTOCOL_VERSION and 'protocol.version' are set, the higher
value wins.

For usage within tests, these semantics are too complex.  Instead,
always use the value from protocol.version configuration when it is
set, falling back to GIT_TEST_PROTOCOL_VERSION.  This way, the envvar
provides a reliable preview of what will happen if the default
protocol version is changed.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>