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