Avoid computing expensive default values when the value is overridden anyways

The most expensive call at the moment is repo.get_changed_files, which
does down the drain if:
- another default_fn overrides the value
- an explicit override is given when creating the `Parameters`

With this change, the default function can return a function as a value,
which is not evaluated unless necessary.

Fixes #616

Author: glandium

src/taskgraph/parameters.py

@@ -4,6 +4,7 @@
 
 import gzip
 import hashlib
+import inspect
 import json
 import os
 import time
@@ -110,7 +111,7 @@ def _get_defaults(repo_root=None):
         "do_not_optimize": [],
         "enable_always_target": True,
         "existing_tasks": {},
-        "files_changed": repo.get_changed_files("AM"),
+        "files_changed": lambda: repo.get_changed_files("AM"),
         "filters": ["target_tasks_method"],
         "head_ref": repo.branch or repo.head_rev,
         "head_repository": repo_url,
@@ -210,7 +211,7 @@ def _fill_defaults(repo_root=None, **kwargs):
 
         for name, default in defaults.items():
             if name not in kwargs:
-                kwargs[name] = default
+                kwargs[name] = default() if inspect.isfunction(default) else default
         return kwargs
 
     def check(self):