Merge branch 'jt/test-protocol-version'

Merge branch 'jt/test-protocol-version'

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