Merge branch 'ab/userdiff-tests'

A bit of code clean-up and a lot of test clean-up around userdiff
area.

* ab/userdiff-tests:
  blame tests: simplify userdiff driver test
  blame tests: don't rely on t/t4018/ directory
  userdiff: remove support for "broken" tests
  userdiff tests: list builtin drivers via test-tool
  userdiff tests: explicitly test "default" pattern
  userdiff: add and use for_each_userdiff_driver()
  userdiff style: normalize pascal regex declaration
  userdiff style: declare patterns with consistent style
  userdiff style: re-order drivers in alphabetical order

t/helper/test-tool.c

@@ -73,6 +73,7 @@ static struct test_cmd cmds[] = {
 	{ "submodule-nested-repo-config", cmd__submodule_nested_repo_config },
 	{ "subprocess", cmd__subprocess },
 	{ "trace2", cmd__trace2 },
+	{ "userdiff", cmd__userdiff },
 	{ "urlmatch-normalization", cmd__urlmatch_normalization },
 	{ "xml-encode", cmd__xml_encode },
 	{ "wildmatch", cmd__wildmatch },