hooks: resolve enlistment root for worktrees outside enlistment tree

[tyrielv]

Summary

All native hooks (virtual-filesystem, read-object, post-index-changed) and managed hooks (GVFS.Hooks) resolved the primary enlistment root by walking up from CWD looking for a .gvfs/ directory. Worktrees placed outside the enlistment directory tree (e.g. in a temp directory or as a sibling) could not use git commands — hooks failed with 'must be run from inside a GVFS enlistment'.

Fix

Added a worktree fallback path: when .gvfs walk-up fails, walk up looking for a .git file (indicating a linked worktree), then resolve the primary enlistment root through the gitdir chain — first from the gvfs-enlistment-root marker file, then by deriving from commondir. The resolved root is validated by checking that .gvfs/ exists.

Native hooks (common.windows.cpp)

• Extract ReadFirstLine, Utf8ToWide, TryParseGitFile helpers
• Refactor GetWorktreePipeSuffix to use shared helpers
• Add TryResolveFromWorktree returning both enlistment root and pipe suffix
• GetGVFSPipeName tries .gvfs walk-up first, then worktree fallback

Managed hooks (Program.cs)

• After TryGetGVFSEnlistmentRoot fails, try TryGetWorktreeInfo → GetEnlistmentRoot() before exiting
• Validate .gvfs exists at the resolved root

Testing

• 3 unit tests for GetEnlistmentRoot (marker file, SharedGitDir fallback, marker-preferred)
• Functional test creating worktree in temp directory, verifying git status from root and subdirectory, file projection, and commits
• All 806 unit tests pass