Abstract hash function usage (#28138)

Refactor Hash interfaces and centralize hash function. This will allow
easier introduction of different hash function later on.

This forms the "no-op" part of the SHA256 enablement patch.

modules/git/object_id_gogit.go

@@ -0,0 +1,28 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+//go:build gogit
+
+package git
+
+import (
+	"github.com/go-git/go-git/v5/plumbing"
+	"github.com/go-git/go-git/v5/plumbing/hash"
+)
+
+func ParseGogitHash(h plumbing.Hash) ObjectID {
+	switch hash.Size {
+	case 20:
+		return ObjectFormatFromID(Sha1).MustID(h[:])
+	}
+
+	return nil
+}
+
+func ParseGogitHashArray(objectIDs []plumbing.Hash) []ObjectID {
+	ret := make([]ObjectID, len(objectIDs))
+	for i, h := range objectIDs {
+		ret[i] = ParseGogitHash(h)
+	}
+
+	return ret
+}