Bug report. Out of memory about git checkout.

[void f <fv729164860@gmail.com>]

—env:

git version: 2.19.1
os : center os
memory 8G

— how to repeat this bug:

Build a repository with large lfs-file use GIT_LFS_SKIP_SMUDGE like
this:  (I can't push a 10G lfs-file to github, So I can’t give you an
example repostiory)

```
hecanwei@MacBook-Pro lfs-test % git st
On branch master
nothing to commit, working tree clean
hecanwei@MacBook-Pro lfs-test % echo "$(cat Xcode_13.4.1.xip )"
version https://git-lfs.github.com/spec/v1
oid sha256:a1e0dbd6d5a96c4a6d3d63600b58486759aa836c2d9f7e8fa6d7da4c7399638b
size 10783587696
```


Rm Xcode_13.4.1.xip

Git checkout .

You will see “Out of memory, realloc failed”

It also use too much memory above version: 2.36.1 macOS


— reason of the bug:


When you execute git checkout, Which have to checkout a lfs-file to
the worktree. Git would execute **convert.c
apply_multi_file_filter()** to convert lfs pointer from git-object to
the lfs file. It will execute a subprocess to convert this file. But
It is strange that git would read all of the file into memory When
finish the git-lfs subprocess. (The code is about pkt-line.c
read_packetized_to_strbut()). Lfs usually is a very large file even
more than the memory. So it would throw out of memory exception.

With this bug, it would have trouble to use sparse-chekout in a
repository with large lfs-file. Because you must init the repository
first and set the sparse-chekout config, than use git
pull/merge/checkout to checkout your subset worktree. It would out of
memory when you checkout it.

I think git don’t need to read all of the file in memory. It can use a
stream to finish the checkout.