Problem:When looking up objects via alternates, the current implementation createsa new ObjectStorage instance for every lookup. This causes severe performancedegradation because each NewObjectStorage() call re-reads the alternates file,re-initializes packfile index structures, and re-loads packfile indexes.For operations that enumerate many objects (push, log, diff), this causes formy test repository with 35K files:- 500+ GB memory allocation (repeatedly loading indexes)- 195+ seconds CPU time for object enumeration- Massive disk I/O (re-reading same packfiles)Benchmarks show git push with alternates is 173x slower than git CLIon a repository with 35K files.Solution:Cache ObjectStorage instances for alternate repositories using sync.Oncefor thread-safe lazy initialization. The cached instances share the sameobjectCache to maximize cache hits across all storages.This fix also extends alternates support to HasEncodedObject andEncodedObjectSize methods which previously did not check alternates.Performance impact:- Object lookups via alternates: ~100x faster- Memory usage: ~500x reduction per lookup- Push operation on 35k file repo: 287s to ~20sSigned-off-by: Cedric BAIL <cedric.bail@appdirect.com>

Add comprehensive tests for the alternates caching functionality:- TestObjectStorageMultipleAlternates: verify multiple alternates work- TestObjectStorageAlternatesHasEncodedObject: verify HasEncodedObject- TestObjectStorageAlternatesEncodedObjectSize: verify EncodedObjectSizeSigned-off-by: Cedric BAIL <cedric.bail@appdirect.com>

Add benchmarks to measure and prevent regression in alternatesperformance using b.Run sub-benchmarks and modern b.Loop():- BenchmarkAlternatesObjectLookup/EncodedObject- BenchmarkAlternatesObjectLookup/HasEncodedObject- BenchmarkAlternatesObjectLookup/EncodedObjectSizeThese benchmarks document the expected performance characteristicsand help catch regressions in future changes.Note: PlainClone with Shared:true cannot be used here due to importcycle (repository.go imports storage/filesystem), so alternatesare set up manually following the same pattern.Signed-off-by: Cedric BAIL <cedric.bail@appdirect.com>