Merge branch 'bc/t5607-avoid-broken-test-fail-prereqs'

The current implementation of GIT_TEST_FAIL_PREREQS is broken in
that checking for the lack of a prerequisite would not work.  Avoid
the use of "if ! test_have_prereq X" in a test script.

* bc/t5607-avoid-broken-test-fail-prereqs:
  t5607: avoid using prerequisites to select algorithm