fix: ensure GraphConfig.root_dir is an absolute path

Author: bhearsum

src/taskgraph/config.py

@@ -157,6 +157,9 @@ def validate_graph_config(config):
 
 
 def load_graph_config(root_dir):
+    # ensure we have an absolute path; this is required for assumptions
+    # made later, such as the `vcs_root` being a directory above `root_dir`
+    root_dir = os.path.abspath(root_dir)
     config_yml = os.path.join(root_dir, "config.yml")
     if not os.path.exists(config_yml):
         raise Exception(f"Couldn't find taskgraph configuration: {config_yml}")