This PR fixes repository discovery when the input path includes non-existent descendants inside an existing repository tree (issue #6383).

Problem

git_repository_discover currently fails early when start_path cannot be prettified (for example, /path/to/repo/missing1/missing2), even if a valid repository exists in an ancestor directory.

What changed

src/libgit2/repository.c

In find_repo_traverse:

• Updated the git_fs_path_prettify(&path, start_path, NULL) handling:
  • If it fails with GIT_ENOTFOUND, discovery now continues using the raw start_path instead of returning immediately.
  • Non-GIT_ENOTFOUND errors are still returned.
• Added a stop condition for relative traversal at "." (when no minimum iterations remain), to avoid walking past the effective root in that case.
• Kept existing traversal/validation behavior for .git dir/file checks while allowing discovery from non-existent subpaths.
• Added a final git_fs_path_prettify_dir(&out->gitdir, out->gitdir.ptr, NULL) after successful discovery.

Why the final prettify_dir call is needed

If the initial prettify fails because the input path is invalid/non-existent, we may still successfully discover a valid repo by walking ancestors. In that case, we should still normalize the discovered gitdir before returning.

That is why git_fs_path_prettify_dir is called at the end with the known-valid discovered gitdir path.

I considered tracking this with a boolean (set when initial prettify fails) and only calling prettify_dir conditionally, but the extra call is low-cost and keeping it unconditional keeps control flow simpler.

tests/libgit2/repo/discover.c

Added tests covering:

• Success when discovering from a non-existent subpath inside an existing repository.
• Failure when starting from a non-existent branch whose parent chain does not reach a repository.
• Failure for a relative non-repo path with "." as ceiling.

Also added test fixture setup for a non-repository folder subtree used by the new cases.

Validation

• Added/updated tests for the new behavior.
• Ran the full test suite; all tests pass.