Help developers by making it easier to run most of the tests under
different versions of over-the-wire protocols.
* jt/test-protocol-version:
t5552: compensate for v2 filtering ref adv.
tests: fix protocol version for overspecifications
t5700: only run with protocol version 1
t5512: compensate for v0 only sending HEAD symrefs
t5503: fix overspecification of trace expectation
tests: always test fetch of unreachable with v0
t5601: check ssh command only with protocol v0
tests: define GIT_TEST_PROTOCOL_VERSION