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