Generalize HEAD to refs

ugit/base.py

@@ -83,7 +83,7 @@ def read_tree(tree_oid):
 def commit(message):
     commit = f"tree {write_tree()}\n"
 
-    HEAD = data.get_HEAD()
+    HEAD = data.get_ref("HEAD")
     if HEAD:
         commit += f"parent {HEAD}\n"
 
@@ -92,7 +92,7 @@ def commit(message):
 
     oid = data.hash_object(commit.encode(), "commit")
 
-    data.set_HEAD(oid)
+    data.update_ref("HEAD", oid)
 
     return oid
 
@@ -100,7 +100,7 @@ def commit(message):
 def checkout(oid):
     commit = get_commit(oid)
     read_tree(commit.tree)
-    data.set_HEAD(oid)
+    data.update_ref("HEAD", oid)
 
 
 Commit = namedtuple("Commit", ["tree", "parent", "message"])