Unify hashing for avatar (#22289)

- Unify the hashing code for repository and user avatars into a
function.
- Use a sane hash function instead of MD5.
- Only require hashing once instead of twice(w.r.t. hashing for user
avatar).
- Improve the comment for the hashing code of why it works.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Yarden Shoham <hrsi88@gmail.com>

services/repository/avatar.go

@@ -5,7 +5,6 @@ package repository
 
 import (
 	"context"
-	"crypto/md5"
 	"fmt"
 	"image/png"
 	"io"
@@ -27,7 +26,7 @@ func UploadAvatar(repo *repo_model.Repository, data []byte) error {
 		return err
 	}
 
-	newAvatar := fmt.Sprintf("%d-%x", repo.ID, md5.Sum(data))
+	newAvatar := avatar.HashAvatar(repo.ID, data)
 	if repo.Avatar == newAvatar { // upload the same picture
 		return nil
 	}