Git中ALTERNATE_DB_ENVIRONMENT环境变量设置备用对象目录失效求助

根据Git源码中object-file.c的注释，设置ALTERNATE_DB_ENVIRONMENT环境变量应该可以替代.git/objects/info/alternates文件来配置备用对象目录。注释内容如下：

The elements on this list come from non-empty elements from colon separated ALTERNATE_DB_ENVIRONMENT environment variable, and $GIT_OBJECT_DIRECTORY/info/alternates, whose contents is similar to that environment variable but can be LF separated.
Its base points at a statically allocated buffer that contains "/the/directory/corresponding/to/.git/objects/...", while its name points just after the slash at the end of ".git/objects/" in the example above, and has enough space to hold all hex characters of the object ID, an extra slash for the first level indirection, and the terminating NUL.

但实际测试中，该环境变量无法生效，而使用.git/objects/info/alternates文件的方法可以正常工作。

复现步骤 #

创建备用对象数据库 #

cd $HOME
mkdir -p .cache/git
cd .cache/git
git init --bare
git remote add bdwgc https://github.com/ivmai/bdwgc
git fetch --all

使用ALTERNATE_DB_ENVIRONMENT环境变量的失效方法 #

export ALTERNATE_DB_ENVIRONMENT=$HOME/.cache/git/objects
cd /tmp
mkdir bdwgc
cd bdwgc
git init
git fetch https://github.com/ivmai/bdwgc

使用.git/objects/info/alternates文件的有效方法 #

cd /tmp
mkdir bdwgc
cd bdwgc
git init
echo "$HOME/.cache/git/objects" >> .git/objects/info/alternates
git fetch https://github.com/ivmai/bdwgc

注：两种方式均使用绝对路径，例如/home/nfl/.cache/git/objects

请问我哪里操作有误？

内容的提问来源于stack exchange，提问作者nfl