GitBlobstore: add cache

[coffeegoddd]

Make GitBlobstore cache fetches.

[github-actions]

This PR is being tested for performance. Please allow ~12 mins for this to complete. If this PR does not result in a performance regression, the performance_approved label will be automatically added to this PR.

[Copilot]

Pull request overview

This PR introduces snapshot-consistent reads for GitBlobstore to ensure that NBS conjoin operations see a consistent view of the underlying git repository even when concurrent remote updates occur. The implementation adds a new Snapshotter interface that GitBlobstore implements, providing read-only snapshots that pin a specific git commit and its path-to-OID index.

Changes:

• Added Snapshotter and ReadOnlySnapshot interfaces to the blobstore package for snapshot-consistent read operations
• Implemented Snapshot() method on GitBlobstore that creates a read-only view pinned to a specific git commit with cached path index
• Updated NBS blobstorePersister.ConjoinAll() to use snapshots when available, ensuring consistent reads during the conjoin operation
• Refactored syncForRead into syncForRead and syncForReadLocked to support snapshot creation under existing lock

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go/store/blobstore/blobstore.go	Defines new Snapshotter and ReadOnlySnapshot interfaces for snapshot-consistent reads
go/store/blobstore/git_blobstore.go	Refactors sync logic to support snapshot creation under lock, adds snapshot index caching fields
go/store/blobstore/git_blobstore_snapshot.go	Implements snapshot functionality with git ls-tree index parsing and read operations
go/store/nbs/bs_persister.go	Updates ConjoinAll to use snapshots for consistent reads, adds blobstoreReader interface
go/store/blobstore/git_blobstore_test.go	Adds comprehensive test verifying snapshot stability across remote advances
go/store/nbs/conjoiner_test.go	Adds integration test for conjoin with concurrent remote updates

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[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
e530687	ok	5937471

version	total_tests
e530687	5937471

correctness_percentage
100.0

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
e990bb6	ok	5937471

version	total_tests
e990bb6	5937471

correctness_percentage
100.0

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
77b1683	ok	5937471

version	total_tests
77b1683	5937471

correctness_percentage
100.0

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
6953aac	ok	5937471

version	total_tests
6953aac	5937471

correctness_percentage
100.0