[PATCH 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         | 3 +++
 2 files changed, 7 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..c719e08414 100644
--- a/t/README
+++ b/t/README
@@ -348,6 +348,9 @@ GIT_TEST_MULTI_PACK_INDEX=<boolean>, when true, forces the multi-pack-
 index to be written after every 'git repack' command, and overrides the
 'core.multiPackIndex' setting to true.
 
+GIT_TEST_REBASE_USE_BUILTIN<boolean>, when false, disables the builtin
+version of git-rebase. See 'rebase.useBuiltin' in git-config(1).
+
 Naming Tests
 ------------
 
-- 
2.19.1.1182.g4ecb1133ce