backend: pass hash to the file system backend

In order to read packs and indices, we need to know the hash algorithm
in use, since the length of the hash will differ.  Pass the hash
algorithm to the file system backend instantiation function.  Since
we're moving to a new major version and can break compatibility, let's
also just turn the two constructors into one that takes all of the
required arguments.  If the user does not want alternates, they can pass
an empty string and we'll do the right thing.

Author: bk2204

backend.go

@@ -2,6 +2,7 @@ package gitobj
 
 import (
 	"bufio"
+	"hash"
 	"io"
 	"os"
 	"path"
@@ -13,16 +14,12 @@ import (
 	"github.com/git-lfs/gitobj/v2/storage"
 )
 
-// NewFilesystemBackend initializes a new filesystem-based backend.
-func NewFilesystemBackend(root, tmp string) (storage.Backend, error) {
-	return NewFilesystemBackendWithAlternates(root, tmp, "")
-}
-
-// NewFilesystemBackendWithAlternates initializes a new filesystem-based
-// backend, optionally with additional alternates as specified in the
+// NewFilesystemBackend initializes a new filesystem-based backend,
+// optionally with additional alternates as specified in the
 // `alternates` variable. The syntax is that of the Git environment variable
-// GIT_ALTERNATE_OBJECT_DIRECTORIES.
-func NewFilesystemBackendWithAlternates(root, tmp, alternates string) (storage.Backend, error) {
+// GIT_ALTERNATE_OBJECT_DIRECTORIES.  The hash algorithm used is specified by
+// the algo parameter.
+func NewFilesystemBackend(root, tmp, alternates string, algo hash.Hash) (storage.Backend, error) {
 	fsobj := newFileStorer(root, tmp)
 	packs, err := pack.NewStorage(root)
 	if err != nil {

object_db.go

@@ -79,7 +79,7 @@ func FromFilesystem(root, tmp string, setters ...Option) (*ObjectDatabase, error
 		setter(args)
 	}
 
-	b, err := NewFilesystemBackendWithAlternates(root, tmp, args.alternates)
+	b, err := NewFilesystemBackend(root, tmp, args.alternates, hasher(args.objectFormat))
 	if err != nil {
 		return nil, err
 	}