Generalize HEAD to refs

ugit/data.py

@@ -10,13 +10,13 @@ def init():
     Path.mkdir(f"{GIT_DIR}/objects")
 
 
-def set_HEAD(oid):
-    with open(f"{GIT_DIR}/HEAD", "w") as f:
+def update_ref(ref, oid):
+    with open(f"{GIT_DIR}/{ref}", "w") as f:
         f.write(oid)
 
 
-def get_HEAD():
-    if Path.is_file(f"{GIT_DIR}/HEAD"):
+def get_ref(ref):
+    if Path.is_file(f"{GIT_DIR}/{ref}"):
         with open(f"{GIT_DIR}/HEAD") as f:
             return f.read().strip()