index: match git on invalid checkout paths

[jelmer]

The NTFS path-element validator rejected any element containing ':' or '', so files with a colon in their name (e.g. with:colon.txt) were silently dropped on clone. Reject only the '.git'/'git~1' alternate-data- stream spellings instead, like git: a bare colon is accepted, and a backslash is only rejected as a separator on Windows while still terminating the .git scan on every platform.

When a tree entry's path is genuinely invalid, abort the checkout rather than silently skipping the entry.

Fixes #2205

[abderrahim]

This seems to fix the issue, thanks.

However, the dulwich CLI crashes with scary backtrace with a bad repo.

❯ git clone bad/ cloned-git
Cloning into 'cloned-git'...
done.
error: invalid path 'test\.git'
fatal: unable to checkout working tree
warning: Clone succeeded, but checkout failed.
You can inspect what was checked out with 'git status'
and retry with 'git restore --source=HEAD :/'

❯ dulwich clone bad/ cloned-dul
counting objects: 6, done.
found 0 deltas to reuse
unable to checkout working tree: invalid path 'test\\.git'
Traceback (most recent call last):
  File "/home/abderrahim/.local/share/uv/tools/dulwich/lib/python3.13/site-packages/dulwich/cli.py", line 2121, in run
    porcelain.clone(
    ~~~~~~~~~~~~~~~^
        parsed_args.source,
        ^^^^^^^^^^^^^^^^^^^
    ...<8 lines>...
        ssh_command=_ssh_command_from_env(),
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/abderrahim/.local/share/uv/tools/dulwich/lib/python3.13/site-packages/dulwich/porcelain/__init__.py", line 1576, in clone
    repo = client.clone(
        path,
    ...<9 lines>...
        protocol_version=protocol_version,
    )
  File "/home/abderrahim/.local/share/uv/tools/dulwich/lib/python3.13/site-packages/dulwich/client.py", line 3290, in clone
    target.get_worktree().reset_index(config=target.get_config_stack())
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/abderrahim/.local/share/uv/tools/dulwich/lib/python3.13/site-packages/dulwich/worktree.py", line 841, in reset_index
    return build_index_from_tree(
        self.path,
    ...<6 lines>...
        blob_normalizer=blob_normalizer,
    )
  File "/home/abderrahim/.local/share/uv/tools/dulwich/lib/python3.13/site-packages/dulwich/index.py", line 2179, in build_index_from_tree
    raise InvalidPathError(entry.path)
dulwich.index.InvalidPathError: invalid path 'test\\.git'