Re: [PATCH v3 1/4] test-lib.sh: introduce and use $_EMPTY_TREE

> This is a special SHA1. Let's keep it at one place, easier to replace
> later when the hash change comes, easier to recognize. Start with an
> underscore to reduce the chances somebody may override it without
> realizing it's predefined.
>
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>  t/t0000-basic.sh                |  2 +-
>  t/t1100-commit-tree-options.sh  |  2 +-
>  t/t4010-diff-pathspec.sh        |  6 ++----
>  t/t4054-diff-bogus-tree.sh      | 10 ++++------
>  t/t5504-fetch-receive-strict.sh |  4 ++--
>  t/test-lib.sh                   |  4 +++-
>  6 files changed, 13 insertions(+), 15 deletions(-)
>
> diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
> index 60811a3..48214e9 100755
> --- a/t/t0000-basic.sh
> +++ b/t/t0000-basic.sh
> @@ -834,7 +834,7 @@ test_expect_success 'git write-tree should be able to write an empty tree' '
>  '
>  
>  test_expect_success 'validate object ID of a known tree' '
> -	test "$tree" = 4b825dc642cb6eb9a060e54bf8d69288fbee4904
> +	test "$tree" = $_EMPTY_TREE
>  '