setup: fix incomplete implementation of Command

`test-install.t` fails without the `get_outputs` method being implemented,
which is used when, `self.report` is `True`.

When 8d7eaff92f9c introduced this change, they probably ran `test-install.t`
without `HGTESTS_ALLOW_NETIO=1`, which does not trigger this codepath.

Differential Revision: https://phab.mercurial-scm.org/D12482

setup.py

@@ -1110,12 +1110,16 b' class hginstallcompletion(Command):'
 
     def initialize_options(self):
         self.install_dir = None
+        self.outputs = []
 
     def finalize_options(self):
         self.set_undefined_options(
             'install_data', ('install_dir', 'install_dir')
         )
 
+    def get_outputs(self):
+        return self.outputs
+
     def run(self):
         for src, dir_path, dest in (
             (
@@ -1127,9 +1131,10 b' class hginstallcompletion(Command):'
         ):
             dir = os.path.join(self.install_dir, *dir_path)
             self.mkpath(dir)
-            self.copy_file(
-                os.path.join('contrib', src), os.path.join(dir, dest)
-            )
+
+            dest = os.path.join(dir, dest)
+            self.outputs.append(dest)
+            self.copy_file(os.path.join('contrib', src), dest)
 
 
 # virtualenv installs custom distutils/__init__.py and