Add PoC for on-demand hooks repo download

This makes pre-commit skip all config entries not listing certain hook

Resolves #637

pre_commit/runner.py

@@ -15,20 +15,26 @@ class Runner(object):
     repository under test.
     """
 
-    def __init__(self, git_root, config_file, store_dir=None):
+    def __init__(self, git_root, config_file, store_dir=None,
+                 filter_repos=False, filter_hook=None):
         self.git_root = git_root
         self.config_file = config_file
         self._store_dir = store_dir
+        self._filter_repos = filter_repos
+        self._filter_hook = filter_hook
 
     @classmethod
-    def create(cls, config_file):
+    def create(cls, config_file, filter_repos=False, filter_hook=None):
         """Creates a Runner by doing the following:
             - Finds the root of the current git repository
             - chdir to that directory
         """
         root = git.get_root()
         os.chdir(root)
-        return cls(root, config_file)
+        return cls(
+            root, config_file,
+            filter_repos=filter_repos, filter_hook=filter_hook
+        )
 
     @cached_property
     def git_dir(self):
@@ -45,8 +51,15 @@ class Runner(object):
     @cached_property
     def repositories(self):
         """Returns a tuple of the configured repositories."""
-        repos = self.config['repos']
-        return tuple(Repository.create(x, self.store) for x in repos)
+        filtered_repos = repos = self.config['repos']
+        if self._filter_repos and self._filter_hook:
+            filtered_repos = (
+                r for r in repos
+                if self._filter_hook in (
+                    h['id'] for h in r['hooks']
+                )
+            )
+        return tuple(Repository.create(x, self.store) for x in filtered_repos)
 
     def get_hook_path(self, hook_type):
         return os.path.join(self.git_dir, 'hooks', hook_type)