sysroot: Move ostree_sysroot_origin_new_from_refspec here

Rather than having it live in admin.  This is useful for other
consumers like the test suite.

src/libostree/ostree-sysroot.c

@@ -1095,3 +1095,18 @@ ostree_sysroot_get_merge_deployment (OstreeSysroot     *self,
   return NULL;
 }
 
+/**
+ * ostree_sysroot_origin_new_from_refspec:
+ * @refspec: A refspec
+ *
+ * Returns: (transfer full): A new config file which sets @refspec as an origin
+ */
+GKeyFile *
+ostree_sysroot_origin_new_from_refspec (OstreeSysroot  *sysroot,
+                                        const char     *refspec)
+{
+  GKeyFile *ret = g_key_file_new ();
+  g_key_file_set_string (ret, "origin", "refspec", refspec);
+  return ret;
+}
+