Use git ls-files -s instead of ls-tree for full-tree enumeration

[tyrielv]

Summary

When no previous commit exists to diff against (sourceTreeSha == null), DiffHelper.PerformDiff runs git ls-tree -r -t HEAD to enumerate all blobs and trees. This walks every tree object — very slow on large repos.

Replace with git ls-files -s, which reads the git index instead of walking tree objects. The index is already materialized in GVFS-mounted repos, making this significantly faster.

The optimization only applies when the target tree matches HEAD (i.e., the index reflects the tree we need). This is always the case for gvfs prefetch, which resolves HEAD as its target (PrefetchVerb.LoadBlobPrefetchArgs → RevParse(HEAD)). For other callers like FastFetch force-checkout (which can target a non-HEAD commit), the code falls back to ls-tree to preserve correctness.

Benchmark (repo with ~2.5M files)

Approach	Time	Speedup
git ls-tree -r -t HEAD (before)	~24s	baseline
git ls-files -s (after)	~6.5s	3.7×
libgit2 in-process tree walk	~8.2s	2.9×
libgit2 in-process index read	~12.9s	1.9×

Also benchmarked libgit2 alternatives: in-process recursive tree walk (2.9× faster than ls-tree) and in-process index read (1.9× — marshaling overhead). git ls-files -s was the fastest and simplest option.

Changes

• GitProcess.cs — New LsFilesStaging() method that runs git ls-files -s
• DiffTreeResult.cs — New ParseFromLsFilesStagingLine() parser for the <mode> <sha> <stage>\t<path> format
• DiffHelper.cs — PerformDiff now uses ls-files -s when sourceTreeSha == null and targetTreeSha matches HEAD's tree (verified via libgit2). Falls back to ls-tree otherwise.
• DiffTreeResultTests.cs — 7 new unit tests for the parser

Safety

• TargetMatchesHeadTree() resolves HEAD's tree SHA via libgit2 and compares to the requested targetTreeSha. Only uses the index-based path when they match.
• Falls back to ls-tree if the index is unavailable, HEAD can't be resolved, or the target differs from HEAD.
• ls-files -s only returns file entries (not tree entries). Tree entries from ls-tree were only used for directory creation, which FlushStagedQueues handles from file paths anyway.