Use git_odb_exists instead of git_revparse_single for ObjectExists

[tyrielv]

Summary

Replaces the heavyweight git_revparse_single call in LibGit2Repo.ObjectExists with git_odb_exists, a purpose-built existence check that skips revparse expression parsing and git_object handle allocation.

Benchmark (os.2020 enlistment, 59.7M objects, 14 packs)

Scenario	Before (revparse)	After (odb_exists)	Speedup
Existing objects	~800 ns/op	~800 ns/op	~1x
Missing objects	2,800,000 ns/op	1,320,000 ns/op	2.1x

Implementation

• Add P/Invoke bindings for git_repository_odb, git_odb_exists, git_odb_free, git_oid_fromstr
• ODB handle lazily acquired on first ObjectExists call, freed in Dispose
• Falls back to git_revparse_single if ODB handle acquisition fails

Semantic split: ObjectExists vs ObjectCanBeParsed

git_odb_exists only checks whether an OID is present in the object store -- it does not decompress or validate the object. This changes behavior for LooseObjectsStep.ClearCorruptLooseObjects, which relied on ObjectExists returning false for corrupt-but-present loose objects to trigger cleanup.

To preserve that behavior, this PR introduces ObjectCanBeParsed(sha) which retains the old git_revparse_single path for corruption detection. LooseObjectsStep is updated to call ObjectCanBeParsed instead of ObjectExists.

Method	Implementation	Use case
ObjectExists	git_odb_exists (fast)	Existence checks -- prefetch, diff walk, commit verification
ObjectCanBeParsed	git_revparse_single (slow but validates)	Corruption detection -- loose object cleanup

Impact

All ObjectExists callers benefit automatically:

• FindBlobsStage (blob prefetch)
• EnumerateMissingTreeEntries (diff/tree walk)
• CommitAndRootTreeExists (commit verification)

LooseObjectsStep preserves existing corruption-detection semantics via ObjectCanBeParsed.

Files changed

• LibGit2Repo.cs -- new P/Invokes, ObjectExists uses git_odb_exists, new ObjectCanBeParsed
• GitRepo.cs -- expose ObjectCanBeParsed through the invoker
• LooseObjectsStep.cs -- call ObjectCanBeParsed instead of ObjectExists