tests: add a special setup where stash.useBuiltin is off

Add a GIT_TEST_STASH_USE_BUILTIN=false test mode which is equivalent
to running with stash.useBuiltin=false. This is needed to spot that
we're not introducing any regressions in the legacy stash version
while we're carrying both it and the new built-in version.

This imitates the equivalent treatment for the built-in rebase in
62c2393 (tests: add a special setup where rebase.useBuiltin is off,
2018-11-14).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/stash.c

@@ -1520,7 +1520,10 @@ static int use_builtin_stash(void)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
 	struct strbuf out = STRBUF_INIT;
-	int ret;
+	int ret, env = git_env_bool("GIT_TEST_STASH_USE_BUILTIN", -1);
+
+	if (env != -1)
+		return env;
 
 	argv_array_pushl(&cp.args,
 			 "config", "--bool", "stash.usebuiltin", NULL);

t/README

@@ -383,6 +383,10 @@ GIT_TEST_REBASE_USE_BUILTIN=<boolean>, when false, disables the
 builtin version of git-rebase. See 'rebase.useBuiltin' in
 git-config(1).
 
+GIT_TEST_STASH_USE_BUILTIN=<boolean>, when false, disables the
+built-in version of git-stash. See 'stash.useBuiltin' in
+git-config(1).
+
 GIT_TEST_INDEX_THREADS=<n> enables exercising the multi-threaded loading
 of the index for the whole test suite by bypassing the default number of
 cache entries and thread minimums. Setting this to 1 will make the