Write pseudorefs through ref backends.

Pseudorefs store transient data in in the repository. Examples are HEAD,
CHERRY_PICK_HEAD, etc.

These refs have always been read through the ref backends, but they were written
in a one-off routine that wrote an object ID or symref directly into
.git/<pseudo_ref_name>.

This causes problems when introducing a new ref storage backend. To remedy this,
extend the ref backend implementation with a write_pseudoref_fn and
update_pseudoref_fn.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/refs-internal.h

@@ -556,6 +556,21 @@ typedef int copy_ref_fn(struct ref_store *ref_store,
 			  const char *oldref, const char *newref,
 			  const char *logmsg);
 
+typedef int write_pseudoref_fn(struct ref_store *ref_store,
+			       const char *pseudoref,
+			       const struct object_id *oid,
+			       const struct object_id *old_oid,
+			       struct strbuf *err);
+
+/*
+ * Deletes a pseudoref. Deletion always succeeds (even if the pseudoref doesn't
+ * exist.), except if old_oid is specified. If it is, it can fail due to lock
+ * failure, failure reading the old OID, or an OID mismatch
+ */
+typedef int delete_pseudoref_fn(struct ref_store *ref_store,
+				const char *pseudoref,
+				const struct object_id *old_oid);
+
 /*
  * Iterate over the references in `ref_store` whose names start with
  * `prefix`. `prefix` is matched as a literal string, without regard
@@ -655,6 +670,9 @@ struct ref_storage_be {
 	rename_ref_fn *rename_ref;
 	copy_ref_fn *copy_ref;
 
+	write_pseudoref_fn *write_pseudoref;
+	delete_pseudoref_fn *delete_pseudoref;
+
 	ref_iterator_begin_fn *iterator_begin;
 	read_raw_ref_fn *read_raw_ref;