From: ZheNing Hu <adlternative@gmail.com>

Although we already had a `--filter=sparse:oid=<oid>` which
can used to clone a repository with limited objects which meet
filter rules in the file corresponding to the <oid> on the git
server. But it can only read filter rules which have been record
in the git server before.

But sometimes users may want to pass their own filter rules
without upload it to the git server first. So now introduce a
new filter `sparse:buffer=<spec>`, which can pass users
required filterspec.

Signed-off-by: ZheNing Hu <adlternative@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.1316.git.1659958159193.gitgitgadget@gmail.com