Merge branch 'tr/test-v-and-v-subtest-only'

Allows N instances of tests run in parallel, each running 1/N parts
of the test suite under Valgrind, to speed things up.

* tr/test-v-and-v-subtest-only:
  perf-lib: fix start/stop of perf tests
  test-lib: support running tests under valgrind in parallel
  test-lib: allow prefixing a custom string before "ok N" etc.
  test-lib: valgrind for only tests matching a pattern
  test-lib: verbose mode for only tests matching a pattern
  test-lib: self-test that --verbose works
  test-lib: rearrange start/end of test_expect_* and test_skip
  test-lib: refactor $GIT_SKIP_TESTS matching
  test-lib: enable MALLOC_* for the actual tests

t/valgrind/valgrind.sh

@@ -4,6 +4,9 @@ base=$(basename "$0")
 
 TOOL_OPTIONS='--leak-check=no'
 
+test -z "$GIT_VALGRIND_ENABLED" &&
+exec "$GIT_VALGRIND"/../../"$base" "$@"
+
 case "$GIT_VALGRIND_MODE" in
 memcheck-fast)
 	;;