[PATCH v2 2/2] tests: add a special setup where rebase.useBuiltin is off

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

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

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 builtin/rebase.c | 5 ++++-
 t/README         | 4 ++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 0ee06aa363..68ad8c1149 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -48,7 +48,10 @@ static int use_builtin_rebase(void)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
 	struct strbuf out = STRBUF_INIT;
-	int ret;
+	int ret, env = git_env_bool("GIT_TEST_REBASE_USE_BUILTIN", -1);
+
+	if (env != -1)
+		return env;
 
 	argv_array_pushl(&cp.args,
 			 "config", "--bool", "rebase.usebuiltin", NULL);
diff --git a/t/README b/t/README
index 242497455f..3df5d12e46 100644
--- a/t/README
+++ b/t/README
@@ -339,6 +339,10 @@ for the index version specified.  Can be set to any valid version
 GIT_TEST_PRELOAD_INDEX=<boolean> exercises the preload-index code path
 by overriding the minimum number of cache entries required per thread.
 
+GIT_TEST_REBASE_USE_BUILTIN=<boolean>, when false, disables the
+builtin version of git-rebase. See 'rebase.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
-- 
2.19.1.1182.g4ecb1133ce