Add change 21 instructions

how_to/Change_21.md

@@ -0,0 +1,23 @@
+- tag: Generalize HEAD to refs
+
+As part of implementing `tag`, we'll generalize the way we handle HEAD. If you
+think about it, HEAD and tags are similar. They are both ways for ugit to attach
+a name to an OID. In case of HEAD, the name is hardcoded by ugit; in case of
+tags, the name will be provided by the user. It makes sense to handle them
+similarly in *data.py*.
+
+In *data.py*, let's extend the function `set_HEAD` and `get_HEAD` to
+`update_ref` and `get_ref`. "Ref" is a short for reference, and that's the name
+Git uses. The function will now accept the name of the ref and write/read it as
+a file under *.ugit* directory. Logically, a ref is a named pointer to an object.
+
+The important change is in *data.py*. The rest of the changes just rename some
+functions:
+
+```
+- get_HEAD()    ->   get_ref('HEAD')
+- set_HEAD(oid) ->   update_ref('HEAD', oid)
+```
+
+Note that we didn't change any behaviour of ugit here, this is purely
+refactoring.