t/lib-proto-disable: restore protocol.allow after config tests

t/lib-proto-disable: restore protocol.allow after config tests

The tests for protocol.allow actually set that variable in
the on-disk config, run a series of tests, and then never
clean up after themselves. This means that whatever tests we
run after have protocol.allow=never, which may influence
their results.

In most cases we either exit after running these tests, or
do another round of test_proto(). In the latter case, this happens to
work because:

  1. Tests of the GIT_ALLOW_PROTOCOL environment variable
     override the config.

  2. Tests of the specific config "protocol.foo.allow"
     override the protocol.allow config.

  3. The next round of protocol.allow tests start off by
     setting the config to a known value.

However, it's a land-mine waiting to trap somebody adding
new tests to one of the t581x test scripts. Let's make sure
we clean up after ourselves.

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