Merge branch 'ab/rebase-in-c-escape-hatch'

The recently merged "rebase in C" has an escape hatch to use the
scripted version when necessary, but it hasn't been documented,
which has been corrected.

* ab/rebase-in-c-escape-hatch:
  tests: add a special setup where rebase.useBuiltin is off
  rebase doc: document rebase.useBuiltin

Documentation/config/rebase.txt

+rebase.useBuiltin::
+	Set to `false` to use the legacy shellscript implementation of
+	linkgit:git-rebase[1]. Is `true` by default, which means use
+	the built-in rewrite of it in C.
++
+The C rewrite is first included with Git version 2.20. This option
+serves an an escape hatch to re-enable the legacy version in case any
+bugs are found in the rewrite. This option and the shellscript version
+of linkgit:git-rebase[1] will be removed in some future release.
++
+If you find some reason to set this option to `false` other than
+one-off testing you should report the behavior difference as a bug in
+git.
+
 rebase.stat::
 	Whether to show a diffstat of what changed upstream since the last
 	rebase. False by default.

builtin/rebase.c

@@ -49,7 +49,10 @@ 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);

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