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/tree_entry_nogogit.go

@@ -9,7 +9,7 @@ import "code.gitea.io/gitea/modules/log"
 
 // TreeEntry the leaf in the git tree
 type TreeEntry struct {
-	ID SHA1
+	ID ObjectID
 
 	ptree *Tree