Re-sync libgit2 overlay admin-owner patch with merged upstream (bump to 1.9.4)

[tyrielv]

Summary

Re-syncs the VFSForGit libgit2 vcpkg overlay patch non-elevated-admin-owner.diff with the version that was actually merged to libgit2 main, replacing the buggy version that was copied verbatim from libgit2/libgit2#7200. Also bumps the pinned libgit2 version 1.9.3 → 1.9.4 and refreshes the overlay README provenance/release notes.

Why

The overlay carried the raw PR #7200 diff, which had four bugs that libgit2 maintainers fixed before/at merge:

1. Handle leak — the linked_token HANDLE from GetTokenInformation(TokenLinkedToken) was never CloseHandle'd. Matters for the long-lived GVFS mount service running as SYSTEM.
2. static HANDLE linked_token in git_fs_path_owner_is — not thread-safe and retained a stale handle across calls. Now a per-call local initialized to NULL by the caller.
3. Failure-path pointer bug — on TokenLinkedToken failure the code did linked_token = NULL (nulling the local pointer parameter) instead of *linked_token = NULL (+ CloseHandle).
4. current_user_sid() only called in the CURRENT_USER branch — but the admin-membership branch also reads linked_token, so a caller passing only USER_IS_ADMINISTRATOR would read an uninitialized handle. It is now called unconditionally before either branch.

What merged upstream

PR #7200 was manually merged to main on 2026-06-07. Relevant commits (all touch only src/util/fs_path.c):

• cc477ee — "Support non-elevated admin user check on Windows"
• f9f36a6 — "Extend Win32 safe.directory admin-owner check to also cover non-elevated processes" (the four bug fixes)
• 44c05e5 — "fs: removed unused variable"
• e805a16 — merge commit

The regenerated diff reproduces the net effect of cc477ee + f9f36a6 + 44c05e5 on top of the pinned libgit2 source.

Version bump: 1.9.3 → 1.9.4

• v1.9.4 (2026-05-22) is the latest v1.9.x release; the official vcpkg libgit2 port already pins it.
• src/util/fs_path.c is byte-identical between v1.9.3 and v1.9.4, so the patched region is unaffected and the diff applies cleanly to both.
• Updated vcpkg.json version-semver and the portfile.cmake SHA512.

Release status

The patch is not yet in any libgit2 release (latest is v1.9.4, which predates the merge). It is expected to ship upstream in v2.0 (no ETA); v1.9.x is stated to be the last of the 1.x line. The overlay patch remains necessary until the pinned port moves to a libgit2 that includes it.

Verification

src\scripts\Build.bat Debug rebuilds libgit2 for both x64-windows-static-aot and x64-windows-dynamic triplets:

-- Applying patch dependencies.diff
-- Applying patch non-elevated-admin-owner.diff
...
All requested installations completed successfully

No hunk failures; libgit2 compiles and the full solution builds to an installer. The patched fs_path.c has HANDLE linked_token = NULL; (no static), a CloseHandle in the done: cleanup, CloseHandle(*linked_token); *linked_token = NULL; in the failure path, and current_user_sid() called unconditionally.