`Git._prepare_ref` terminates an object name with a line feed and writes it to
the persistent `git cat-file --batch-check` or `--batch` process, and
`__get_object_header` reads exactly one response line back. A name that itself
contains a line feed is therefore two requests against one response read, and
the stream is left one response behind for the lifetime of the `Git` instance,
so every later lookup is answered with the header of an object it did not ask
for.

`Repo.is_valid_object` hands its argument straight to
`partial_to_complete_sha_hex`, so a single call with an untrusted revision
string is enough to get there. After `repo.is_valid_object("HEAD\nHEAD")`,
`repo.commit(<sha>)` returns a different commit than the one named and
`Repo.odb.info` reports another object's type and size, with nothing to
indicate it. On the `--batch` stream the size from the mismatched header also
misaligns the content reads, so object data is read from the wrong offsets and
a tree read right afterwards fails with `Invalid tree entry mode`.

Git has no way to express an object name containing a line feed on that stdin,
so `_prepare_ref` now refuses one and keeps the single trailing line feed that
is the request terminator. `Repo.is_valid_object` reports such a name as
invalid, which it is, and `short_to_long` keeps turning it into `BadName`.

`IndexFile._write_path_to_stdin` writes paths to `git checkout-index --stdin`
the same way, and unlike an object name a path can legitimately contain a line
feed. With `dir/x\nkeep.txt` in the index, `index.checkout(paths=["dir"],
force=True)` had Git read two paths, so `keep.txt` was checked out over the
local copy while the requested file was never written. Git also unquotes a
line-feed separated path that begins with a double quote, which made a file
named `"q"` impossible to check out. That call site now runs with `-z` and
NUL-terminated paths, which Git does not unquote.

Adds regression tests in `test/test_git.py` and `test/test_index.py`, both of
which fail before this change. The rest of the suite is unaffected on Python
3.11 on macOS, apart from eight failures that predate the change in
`test_commit.py` and `test_submodule.py`; `ruff`, `mypy`,
`basedpyright --warnings` and the docs build are clean.